Long-term nitrogen and phosphorus addition have stronger negative effects on microbial residual carbon in subsoils than topsoils in subtropical forests.

Highly weathered lowland (sub)tropical forests are widely recognized as nitrogen (N)-rich and phosphorus (P)-poor, and the input of N and P affects soil carbon (C) cycling and storage in these ecosystems. Microbial residual C (MRC) plays a crucial role in regulating soil organic C (SOC) stability in forest soils. However, the effects of long-term N and P addition on soil MRC across different soil layers remain unclear. This study conducted a 12-year N and P addition experiment in two typical subtropical plantation forests dominated by Acacia auriculiformis and Eucalyptus urophylla trees, respectively. We measured plant C input (fine root biomass, fine root C, and litter C), microbial community structure, enzyme activity (C/N/P-cycling enzymes), mineral properties, and MRC. Our results showed that continuous P addition reduced MRC in the subsoil (20-40 cm) of both plantations (A. auriculiformis: 28.44% and E. urophylla: 28.29%), whereas no significant changes occurred in the topsoil (0-20 cm). N addition decreased MRC in the subsoil of E. urophylla (25.44%), but had no significant effects on A. auriculiformis. Combined N and P addition reduced MRC (34.63%) in the subsoil of A. auriculiformis but not in that of E. urophylla. The factors regulating MRC varied across soil layers. In the topsoil (0-10 cm), plant C input (the relative contributions to the total variance was 20%, hereafter) and mineral protection (47.2%) were dominant factors. In the soil layer of 10-20 cm, both microbial characteristics (41.3%) and mineral protection (32.3%) had substantial effects, whereas the deeper layer (20-40 cm) was predominantly regulated by microbial characteristics (37.9%) and mineral protection (18.8%). Understanding differential drivers of MRC across soil depth, particularly in deeper soil layers, is crucial for accurately predicting the stability and storage of SOC and its responses to chronic N enrichment and/or increased P limitation in (sub)tropical forests.

Climate warming alters the relative importance of plant root and microbial community in regulating the accumulation of soil microbial necromass carbon in a Tibetan alpine meadow.

Climate warming is predicted to considerably affect variations in soil organic carbon (SOC), especially in alpine ecosystems. Microbial necromass carbon (MNC) is an important contributor to stable soil organic carbon pools. However, accumulation and persistence of soil MNC across a gradient of warming are still poorly understood. An 8-year field experiment with four levels of warming was conducted in a Tibetan meadow. We found that low-level (+0-1.5°C) warming mostly enhanced bacterial necromass carbon (BNC), fungal necromass carbon (FNC), and total MNC compared with control treatment across soil layers, while no significant effect was caused between high-level (+1.5-2.5°C) treatments and control treatments. The contributions of both MNC and BNC to soil organic carbon were not significantly affected by warming treatments across depths. Structural equation modeling analysis demonstrated that the effect of plant root traits on MNC persistence strengthened with warming intensity, while the influence of microbial community characteristics waned along strengthened warming. Overall, our study provides novel evidence that the major determinants of MNC production and stabilization may vary with warming magnitude in alpine meadows. This finding is critical for updating our knowledge on soil carbon storage in response to climate warming.

The priming effects of plant leachates on dissolved organic matter degradation in water depend on leachate type and water stability.

The modification of dissolved organic matter (DOM) degradation by plant carbon inputs represents a critical biogeochemical process that controls carbon dynamics. However, the priming effects (PEs) different plant tissues induce on the degradation of DOM pools with different stabilities remain unknown. In this study, PEs, induced by different tissue leachates of Phragmites australis, were evaluated via changes in DOM components and properties of both fresh and tidal water (with different stabilities). The results showed that DOM derived from different plant tissue leachates differed in composition and bioavailability. Inputs of tissue leachates induced PEs with different intensities and directions (negative or positive) on DOM degradation of fresh and tidal water. In fresh water, the PEs of leaf and root leachates were significantly higher than those of stem and rhizome leachates. The PE direction changed for DOM degradation between fresh and tidal water. The addition of leaf and root leachates tended to induce positive PEs on DOM degradation of fresh water, while resulting in negative PEs on DOM degradation of tidal water. Negative PEs for tidal water DOM may be due to preferential utilization of microbes, high salinity, and/or the promotion of exogenous DOM production from plant tissues. The results indicate that intensity and direction of PEs induced by plant leachates depend on both leachate type and water stability. The findings highlight the necessity to examine the nature of exogenous and native DOM when interpreting the interactive processes that regulate DOM degradation.

Driving factors of soil organic carbon sequestration under straw returning across China's uplands.

Straw returning is suggested as a sustainable agricultural practice to promote soil organic carbon (SOC) sequestration, whose magnitude can be influenced by climatic, edaphic and agronomic factors simultaneously. However, the driving factors regulating straw returning-induced SOC increase in China's uplands remain uncertain. This study conducted a meta-analysis by collecting data from 238 trials at 85 field sites. The results showed that straw returning significantly increased SOC content by an average of 16.1% ± 1.5% with an average sequestration rate of 0.26 ± 0.02 g kg-1 yr-1. The improvement effects were significantly better in the northern China (NE-NW-N) than in the eastern and central (E-C). SOC increases were more pronounced in C-rich and alkaline soils, in cold and dry climates, and under larger amounts of straw-C and moderate nitrogen fertilizer inputs. Longer experimental period resulted in higher SOC increase rates but lower SOC sequestration rates. Furthermore, partial correlation analysis and structural equation modelling revealed that total straw-C input was the key driving factor of SOC increase rate whereas straw returning duration was the dominant limiting factor of SOC sequestration rate across China. Climate conditions were potential limiting factors of SOC increase rate in NE-NW-N and SOC sequestration rate in E-C. It was suggested that straw returning with large application amounts should be more strongly recommended in uplands in NE-NW-N especially in the straw applications at the beginning, from the perspective of SOC sequestration.

Divergent contribution of particulate and mineral-associated organic matter to soil carbon in grassland.

Sequestration of soil organic carbon (SOC) is an effective means to draw atmospheric CO2. Grassland restoration is one of the fastest methods to increase soil C stocks, and particulate-associated C and mineral-associated C play critical roles in soil C stocks during restoration. Herein, we developed a conceptual mechanistic frame regarding the contributions made by mineral-associated organic matter to soil C during the restoration of temperate grasslands. Compared to 1-year grassland restoration, 30-year restoration increased mineral-associated organic C (MAOC) by 41% and particulate organic C (POC) by 47%. The SOC changed from microbial MAOC predominance to plant-derived POC predominance, as the POC was more sensitive to grassland restoration. The POC increased with plant biomass (mainly litter and root biomass), while the increase in MAOC was mainly caused by the combined effects of increasing microbial necromass and leaching of the base cations (Ca-bound C). Plant biomass accounted for 75% of the increase in POC, whereas bacterial and fungal necromass contributed to 58% of the variance in MAOC. POC and MAOC contributed to 54% and 46% of the increase in SOC, respectively. Consequently, the accumulation of the fast (POC) and slow (MAOC) pools of organic matter are important for the sequestration of SOC during grassland restoration. Overall, simultaneous tracing of POC and MAOC helps further understand the mechanisms and predict soil C dynamics combined with the input of plant C, microbial properties, and availability of soil nutrients during grassland restoration.

Labile carbon-induced soil organic matter turnover in a subtropical forest under different redox conditions.

Labile organic carbon (LOC) input strongly affects soil organic matter (SOM) dynamics, including gains and losses. However, it is unclear how redox fluctuations regulate these processes of SOM decomposition and formation induced by LOC input. The objective of this study was to explore the impacts of LOC input on SOM turnover under different redox conditions. Soil samples were collected in a subtropical forest. A single pulse of 13C-labeled glucose (i.e., LOC) was applied to the soil. Soil samples were incubated for 40 days under three redox treatments, including aerobic, anoxic, and 10-day aerobic followed by 10-day anoxic conditions. Results showed that LOC input affected soil priming and 13C-SOM accumulation differently under distinct redox conditions by altering the activities of various microorganisms. 13C-PLFAs (phospholipid fatty acids) were analyzed to determine the role of microbial groups in SOM turnover. Increased activities of fungi and gram-positive bacteria (i.e., the K-strategists) by LOC input could ingest metabolites or residues of the r-strategists (e.g., gram-negative bacteria) to result in positive priming. Fungi could use gram-negative bacteria to stimulate priming intensity via microbial turnover in aerobic conditions first. Reduced activities of K-strategists as a result of the aerobic to anoxic transition decreased priming intensity. The difference in LOC retention in SOM under different redox conditions was mainly attributable to 13C-particulate organic carbon (13C-POC) accumulation. Under aerobic conditions, fungi and gram-positive bacteria used derivatives from gram-negative bacteria to reduce newly formed POC. However, anoxic conditions were not conducive to the uptake of gram-negative bacteria by fungi and gram-positive bacteria, favoring SOM retention. This work indicated that redox-regulated microbial activities can control SOM decomposition and formation induced by LOC input. It is extremely valuable for understanding the contribution of soil affected by redox fluctuations to the carbon cycle.

Younger carbon dominates global soil carbon efflux.

Soil carbon (C) is comprised of a continuum of organic compounds with distinct ages (i.e., the time a C atom has experienced in soil since the C atom entered soil). The contribution of different age groups to soil C efflux is critical for understanding soil C stability and persistence, but is poorly understood due to the complexity of soil C pool age structure and potential distinct turnover behaviors of age groups. Here, we build upon the quantification of soil C transit times to infer the age of C atoms in soil C efflux (aefflux ) from seven sequential soil layer depths down to 2 m at a global scale, and compare this age with radiocarbon-inferred ages of C retained in corresponding soil layers (asoil ). In the whole 0-2 m soil profile, the mean aefflux is 194 21 1021 (mean with 5%-95% quantiles) year and is just about one-eighth of asoil ( 1476 717 2547 year), demonstrating that younger C dominates soil C efflux. With increasing soil depth, both aefflux and asoil are increased, but their disparities are markedly narrowed. That is, the proportional contribution of relatively younger soil C to efflux is decreased in deeper layers, demonstrating that C inputs (new and young) stay longer in deeper layers. Across the globe, we find large spatial variability of the contribution of soil C age groups to C efflux. Especially, in deep soil layers of cold regions (e.g., boreal forests and tundra), aefflux may be older than asoil , suggesting that older C dominates C efflux only under a limited range of conditions. These results imply that most C inputs may not contribute to long-term soil C storage, particularly in upper layers that hold the majority of new C inputs.

Depth-dependent drivers of soil microbial necromass carbon across Tibetan alpine grasslands.

Microbial necromass carbon (C) has been considered an important contributor to persistent soil C pool. However, there still lacks large-scale systematic observations on microbial necromass C in different soil layers, particularly for alpine ecosystems. Besides, it is still unclear whether the relative importance of biotic and abiotic variables such as plant C input and mineral properties in regulating microbial necromass C would change with soil depth. Based on the combination of large-scale sampling along a ~2200 km transect across Tibetan alpine grasslands and biomarker analysis, together with a global data synthesis across grassland ecosystems, we observed a relatively low proportion of microbial-derived C in Tibetan alpine grasslands compared to global grasslands (topsoil: 45.4% vs. 58.1%; subsoil: 41.7% vs. 53.7%). We also found that major determinants of microbial necromass C depended on soil depth. In topsoil, both plant C input and mineral protection exerted dominant effects on microbial necromass C. However, in subsoil, the physico-chemical protection provided by soil clay particles, iron-aluminum oxides, and exchangeable calcium dominantly facilitated the preservation of microbial necromass C. The differential drivers over microbial necromass C between soil depths should be considered in Earth system models for accurately forecasting soil C dynamics and its potential feedback to global warming.

Carbon footprint and net carbon gain of major long-term cropping systems under no-tillage.

Due to increased contribution from agriculture sector to total greenhouse gas emissions, there is need to study the ability of no-tilled diverse cropping systems including crop sequences and bio-covers to mitigate C equivalent emissions. Thus, C-footprint was calculated for a long-term experiment at the University of Tennessee's Research and Education Center in Milan with six-crop sequences: continuous cotton (Gossypium hirsutum L.), cotton-corn (Zea mays L.), continuous corn, corn-soybean (Glycine max L.), continuous soybean, and soybean-cotton interacted with four bio-covers: poultry litter, hairy vetch (Vicia villosa), winter wheat (Triticum aestivum), and fallow control with three replicates in a strip-plot design. During the experiment duration (2002-2017), field inputs (fertilizers, pesticides, and machinery used for planting, chemical applications, and harvesting) and outputs (crop yield, aboveground, and belowground residue) were assessed for each crop sequence/bio-cover combination to calculate total C equivalence of inputs and outputs, net C gain, C footprint per kg yield, sustainability index, and nitrous oxide emissions. For continuous corn, C-based input emissions were significantly higher by 0.28-0.62 Mg CO2 eq. ha-1 yr-1 than all other sequences, however, a greater net C gain (5.4 Mg C eq. ha-1 yr-1) was also observed due to increased crop yield, aboveground and belowground residues. Poultry litter application resulted in lower C-footprint (1.59-2.09 kg CO2 eq. kg-1 yield) than hairy vetch, wheat, and fallow under all crop sequences. Hairy vetch also lowered C-footprint per kg yield (∼2-14%) when compared with wheat under continuous systems of corn, soybean, and cotton, and cotton-corn rotation. Poultry litter application increased sustainability index (23-45) of all cropping sequences compared with other bio-covers. Hairy vetch improved sustainability index of corn including cropping sequences as compared with wheat and fallow. Inclusion of soybean and cotton with corn significantly decreased nitrous oxide emissions by 20-25%. The major factor contributing towards C-based input emissions was N fertilizer with 68% contribution to total emissions on average. It is concluded that application of poultry litter can reduce per yield C-footprint and enhance production system sustainability compared with hairy vetch, wheat, and fallow for monocultures or rotations of corn, soybean, cotton. Additionally, hairy vetch can outperform wheat in reducing the per yield C-footprint for continuous corn/soybean/cotton, and cotton-corn rotation. Especially for corn production systems, hairy vetch can enhance sustainability index compared with wheat and fallow. In order to increase per hectare net C gain, reduce per yield C-footprint and enhance sustainability index simultaneously, integration of continuous corn or corn-soybean/cotton rotation with bio-cover poultry litter or hairy vetch may perform better than the monocultures of soybean or cotton integrated with bio-cover wheat or fallow control in the Mid-south USA.

Response to Comment on 'Soil carbon persistence governed by plant input and mineral protection at regional and global scales'.

We demonstrated that ignoring the non-linear relationship between topsoil Δ14 C and plant carbon (C) input in Wu et al.'s analysis was the fundamental reason for the discrepancy between their analysis and ours. By considering such a non-linear relationship, plant C input still predominantly governs the topsoil C turnover.

Soil extracellular enzyme stoichiometry reveals the nutrient limitations in soil microbial metabolism under different carbon input manipulations.

Changes in the quality and quantity of litter and root inputs due to climate change and human activities can influence below-ground biogeochemical processes in forest ecosystems. However, it is unclear whether and how much aboveground litter and root inputs affect soil microbial metabolism and nutrient limitation mechanisms. In this study, according to a 4-years field manipulation experiment, litter and root manipulations (control (CK), double litter input (DL), no litter (NL), no root (NR), and no inputs (NI)) were set up to analyze the extracellular enzyme activities and stoichiometric ratios characteristics of 0-10 cm and 10-20 cm soils, explore the metabolic limitations of microorganisms, and clarify the main driving factors restricting nutrient limitation. The results showed that the enzyme activities associated with the C cycling (ß-1,4-glucosidase (BG), cellulose disaccharide hydrolase (CBH)) and N cycling (ß-1,4-N-acetylglucosaminidase (NAG), leucine aminopeptidase (LAP)) in DL treatment were significantly higher than those in NR treatment. Moreover, enzyme activities related to P cycling are significantly higher in comparison to other treatments. The acid phosphatase (AP), which is related to the P cycle, showed the highest activity under NR treatment. In addition, there was no significant difference in soil microbial metabolic limitation by the different carbon inputs, which did not change the original nutrient limitation pattern. The main drivers of microbial nutrient metabolic limitation included soil physicochemical properties, soil total nutrients, and available nutrients, among which soil SWC and pH presented the greatest influence on microbial C limitation and soil total nutrients showed the greatest influence on microbial N limitation. Changes in soil carbon input altered soil extracellular enzyme activities and their stoichiometric ratios by affecting soil physicochemical properties, total nutrients. This study provides data for the understanding of material cycling in forest ecosystems under environmental change.

Effect of nitrogen addition on soil net nitrogen mineralization in topsoil and subsoil regulated by soil microbial properties and mineral protection: Evidence from a long-term grassland experiment.

Soil net nitrogen mineralization (Nmin), a microbial-mediated conversion of organic to inorganic N, is critical for grassland productivity and biogeochemical cycling. Enhanced atmospheric N deposition has been shown to substantially increase both plant and soil N content, leading to a major change in Nmin. However, the mechanisms underlying microbial properties, particularly microbial functional genes, which drive the response of Nmin to elevated N deposition are still being discussed. Besides, it is still uncertain whether the relative importance of plant carbon (C) input, microbial properties, and mineral protection in regulating Nmin under continuous N addition would vary with the soil depth. Here, based on a 13-year multi-level field N addition experiment conducted in a typical grassland on the Loess Plateau, we elucidated how N-induced changes in plant C input, soil physicochemical properties, mineral properties, soil microbial community, and the soil Nmin rate (Rmin)-related functional genes drove the responses of Rmin to N addition in the topsoil and subsoil. The results showed that Rmin increased significantly in both topsoil and subsoil with increasing rates of N addition. Such a response was mainly dominated by the rate of soil nitrification. Structural equation modeling (SEM) revealed that a combination of microbial properties (functional genes and diversity) and mineral properties regulated the response of Rmin to N addition at both soil depths, thus leading to changes in the soil N availability. More importantly, the regulatory impacts of microbial and mineral properties on Rmin were depth-dependent: the influences of microbial properties weakened with soil depth, whereas the effects of mineral protection enhanced with soil depth. Collectively, these results highlight the need to incorporate the effects of differential microbial and mineral properties on Rmin at different soil depths into the Earth system models to better predict soil N cycling under further scenarios of N deposition.

Soil organic carbon priming co-regulated by labile carbon input level and long-term fertilization history.

Labile carbon (C) input and fertilization have important consequences for soil organic matter (SOM) decomposition via the priming effect (PE), thereby impacting soil fertility and C sequestration. However, it remains largely uncertain on how the labile C input levels interact with long-term fertilization history to control PE intensity. To clarify this question, soil samples were collected from a 38-year fertilization field experiment (including five treatments: chemical nitrogen fertilizer, N; chemical fertilizer, NPK; manure, M1; 200 % manure, M2; NPK plus M2, NPKM2), with strongly altered soil physiochemical properties (i.e., soil aggregation, organic C and nutrient availability). These soil samples were incubated with three input levels of 13C-glucose (without glucose, control; low, 0.4 % SOC; high, 2.0 % SOC) to clarify the underlying mechanisms of PE. Results showed that the PE significantly increased with glucose input levels, with values increasing from negative or weak (-2.21 to 3.55 mg C g-1 SOC) at low input level to strongly positive (5.62 to 8.57 mg C g-1 SOC) at high input level across fertilization treatments. The increased PE intensity occurred along with decreased dissolved total nitrogen (DTN) contents and increased ratios of dissolved organic C to DTN, implying that the decline in N availability largely increased PE via enhanced microbial N mining from SOM. Compared to N and NPK treatments, the PE was significantly lower in the manure-amendment treatments, especially for low input level, due to more stable SOM by aggregate protection and higher N and phosphorus availability. These results suggested that manure application could alleviate SOM priming via increased soil C stability and nutrient availability. Collectively, our findings emphasize the importance of long-term fertilization-driven changes in labile C inputs, SOM stability, and nutrient availability in regulating PE and soil C dynamics. This knowledge advances our understanding of the long-term fertilization management for soil C sequestration.

Changes in soil microbial biomass and organic C pools improve the sustainability of perennial grass and legume system under organic nutrient management.

Introduction: The perennial grass-legume cropping system benefits soil because of its high biomass turnover, cover cropping nature, and different foraging behaviors. We investigated the response of soil organic carbon (SOC) pools and their stock to organic and inorganic nutrient management in the Guinea grass and legume (cowpea-Egyptian clover) cropping system. Methods: Depth-wise soil samples were collected after harvesting the Egyptian clover. Based on the ease of oxidation with chromic acid, different pools of SOC oxidizable using the Walkley-Black C method, very labile, labile, less labile, non-labile; and dissolved organic C (DOC), microbial biomass C (MBC), and total organic C (TOC) in soils were analyzed for computing several indices of SOC. Result and discussion: After 10 years of crop cycles, FYM and NPKF nutrient management recorded greater DOC, MBC, SOC stocks, and C sequestration than the NPK. Stocks of all SOC pools and carbon management index (CMI) decreased with soil depth. A significant improvement in CMI, stratification ratio, sensitivity indices, and sustainable yield index was observed under FYM and NPKF. This grass-legume intercropping system maintained a positive carbon balance sequestered at about 0.8Mg C ha-1 after 10 years without any external input. Approximately 44-51% of the applied carbon through manure was stabilized with SOC under this cropping system. The DOC, MBC, and SOC in passive pools were identified for predicting dry fodder yield. This study concludes that the application of organics in the perennial grass-legume inter cropping system can maintain long-term sustainability, enhance the C sequestration, and offset the carbon footprint of the farm enterprises.

[Effects of Different Carbon Inputs on Soil Stoichiometry in Tianshan Mountains].

Soil C, N, and P elements are important components of the forest ecosystem. Studying the influence of exogenous carbon input change on the stoichiometry of the forest soil can reveal the element recycling process and the balanced feedback mechanism of the forest ecosystem. In this study, using the research object of a spruce forest in Tianshan Mountain, the short-term effect of exogenous carbon input on soil C, N, and P in the soil was analyzed through Detritus Input and Removal Treatment (DIRT), and then the interrelationship between soil stoichiometry and other soil physicochemical factors under different treatments was discussed. The results showed that:① the soil C, N, and P contents in most soil layers were the highest double litter (DL) treatment, soil ω(C) by soil depth from shallow to deep was 168.92, 119.88, 103.33, and 64.23 g·kg-1; soil ω(N) was 10.60, 9.32, 8.78, and 8.07 g·kg-1; soil ω(P) was 0.50, 0.45, 0.37, and 0.36 g·kg-1; in the no input (NI) treatment, soil ω(C) by soil depth from shallow to deep was 104.56, 89.24, 48.08, and 43.96 g·kg-1; soil ω(N) was 6.83, 2.60, 2.63, and 2.22 g·kg-1; soil ω(P) was 0.40, 0.34, 0.32, and 0.22 g·kg-1; and a decreased trend was shown with the deepening of the soil layer. Except in the NI treatment, C:N was 0-10 cm and significantly higher than that in other soils (P<0.05), NL soil C:P at 30-50 cm was significantly higher than that in other soils, and NI soil N:P was 0-10 cm and significantly higher than that in other soils (P<0.05). ② Microbial carbon, nitrogen, and phosphorus were significantly higher from 0-10 cm than that in other soil layers (P<0.05). ③ Redundancy analysis results showed that soluble organic carbon and microbial nitrogen at different carbon input levels were important factors affecting the stoichiometric characteristics of soil C, N, and P.

Increased soil organic matter after 28 years of nitrogen fertilization only with plastic film mulching is controlled by maize root biomass.

Nitrogen (N) fertilization and plastic film mulching (PFM) are two widely applied management practices for crop production. Both of them impact soil organic matter individually, but their interactive effects as well as the underlying mechanisms are unknown. Soils from a 28-year field experiment with maize monoculture under three levels of N fertilization (0, 135, and 270 kg N ha-1 yr-1) and with or without PFM were analyzed for soil organic C (SOC) content, total soil nitrogen (N), root biomass, enzyme activities, and SOC mineralization rates. After 28 years, N fertilization increased root biomass and consequently, SOC by 26% (averaged across the two fertilizer application rates) and total soil N by 25%. These increases, however, were only in soil with PFM, as PFM reduced N leaching and loss, as a result of a diurnal internal water cycle under the mulch. The SOC mineralization was slower with N fertilization, regardless of the PFM treatment. This trend was attributed to the 43% decrease of ß-glucosidase activity (C cycle enzyme) and 51% drop of leucine aminopeptidase (N cycle) with N fertilization, as a result of a strong decrease in soil pH. In conclusion, root biomass acting as the main source of soil C, resulted in an increase of soil organic matter after 28 year of N fertilization only with PFM.

Perennial Crops Can Compensate for Low Soil Carbon Inputs from Maize in Ley-Arable Systems.

(1) Background: Soil organic carbon (SOC) in agricultural soils plays a crucial role in mitigating global climate change but also, and maybe more importantly, in soil fertility and thus food security. Therefore, the influence of contrasting cropping systems on SOC not only in the topsoil, but also in the subsoil, needs to be understood. (2) Methods: In this study, we analyzed SOC content and δ13C values from a crop rotation experiment for biogas production, established in southern Germany in 2004. We compared two crop rotations, differing in their proportions of maize (0 vs. 50%) and perennial legume-grass leys as main crops (75 vs. 25%). Maize was cultivated with an undersown white clover. Both rotations had an unfertilized variant and a variant that was fertilized with biogas digestate according to the nutrient demand of crops. Sixteen years after the experiment was established, the effects of crop rotation, fertilization, and soil depth on SOC were analyzed. Furthermore, we defined a simple carbon balance model to estimate the dynamics of δ13C in soil. Simulations were compared to topsoil data (0-30 cm) from 2009, 2017, and 2020, and to subsoil data (30-60 cm) from 2020. (3) Results: Crop rotation and soil depth had significant effects, but fertilization had no effect on SOC content and δ13C. SOC significantly differed between the two crop rotations regarding δ13C in both depths but not regarding content. Annual enrichment in C4 (maize) carbon was 290, 34, 353, and 70 kg C ha-1 per maize year in the topsoil and subsoil of the unfertilized and fertilized treatments, respectively. These amounts corresponded to carbon turnover rates of 0.8, 0.3, 0.9, and 0.5% per maize year. Despite there being 50% maize in the rotation, maize carbon only accounted for 20% of the observed carbon sequestration in the topsoil. Even with pre-defined parameter values, the simple carbon model reproduced observed δ13C well. The optimization of model parameters decreased the carbon use efficiency of digestate carbon in the soil, as well as the response of belowground carbon allocation to increased aboveground productivity of maize. (4) Conclusions: Two main findings resulted from this combination of measurement and modelling: (i) the retention of digestate carbon in soil was low and its effect on δ13C was negligible, and (ii) soil carbon inputs from maize only responded slightly to increased above-ground productivity. We conclude that SOC stocks in silage maize rotations can be preserved or enhanced if leys with perennial crops are included that compensate for the comparably low maize carbon inputs.

[Short Term Effects of a Changing Carbon Input on the Soil Respiration of Picea schrenkiana Forests in the Tianshan Mountains, Xinjiang].

The impact of exogenous carbon input changes on forest soil respiration provides the basis for an intensive analysis of the forest carbon cycle. Based on a plant residue addition and removal control experiment, this study investigated the short-term soil respiration response to carbon input changes of Picea schrenkiana on the Tianshan Mountains during their growing season with five different carbon input treatments:control, double litter, no root, no litter, and no input. The results revealed that, during the entire observation period, the cumulative soil respiration rates were 3.38, 3.94, 2.65, 2.87, and 2.01 µmol·(m2·s)-1 in the double litter, control, no litter, no root, and no input treatments, respectively. Compared with the control treatment, the cumulative soil CO2 efflux increased by 402.65 g·m-2 in the double litter treatment, whereas it decreased by 515.00, 354.73, and 967.15 g·m-2 in the no litter, no root, and no input treatments, respectively. The mineral soil respiration, litterfall respiration, and root respiration contributed 59.46%, 21.49%, and 14.79%, respectively, to the total soil respiration rate. PCA analysis revealed that the soil respiration rate was positively correlated with the soil temperature, soil moisture, soil total phosphorus content, pH, and soil organic carbon content, and negatively correlated with the soil bulk density, while the soil total nitrogen content, carbon nitrogen ratio, and soil electrical conductivity had no effect on the soil respiration rate.

[Denitrification Loss and N2O Emission from Different Carbon Inputs in Orchard Drains Sediments].

The orchard is an important economic crop in Taihu Lake region. Heavy nitrogen application in orchard results in great nitrogen loss to drainage ditch, and unbalanced carbon nitrogen ratio. Therefore, carbon might be an important limiting factor for sediment nitrification and denitrification. A soil incubation experiment controlled by the acetylene inhibition method was conducted under laboratory conditions to study the denitrification loss and N2O emissions of orchard drainage ditch soil. We designed five carbon input levels of 0, 5, 25, 50 and 100 mg·L-1, which were noted as C0, C1, C2, C3 and C4, respectively, meanwhile there was 5 mg·L-1 net nitrogen input in the form of KNO3 in each treatment. The results showed that carbon inputs could stimulate both denitrification rates (DN) and N2O emission rates. Carbon and nitrogen ratio had a significant effect on N2O emission rates and denitrification loss rates (P<0.05). When the carbon and nitrogen ratio was 10:1, total cumulative denitrification losses and N2O emissions were both highest (319.26 µg·kg-1 and 6.20 µg·kg-1, respectively) among the treatments, which accounted for 1.28% and 0.02% of net nitrogen input, respectively. This result indicated that the carbon and nitrogen ratio of 10:1 was most favorable for N2O emissions and denitrification process in sediments.