Long-term chemical and organic fertilization induces distinct variations of microbial associations but unanimous elevation of soil multifunctionality.

Intricate microbial associations contribute greatly to the multiple functions (multifunctionality) of natural ecosystems. However, the relationship between microbial associations and soil multifunctionality (SMF) in artificial ecosystems, particularly in agricultural ecosystem with frequent fertilization, remains unclear. In this study, based on a 28-year paddy field experiment, high-throughput sequencing and networks analysis was performed to investigate changes in soil microbial (archaea, bacteria, fungi, and protists) associations and how these changes correlate with SMF under long-term fertilization. Compared to no fertilization (CK), both chemical fertilization with N, P, and K (CF) and chemical fertilization plus rice straw retention (CFR) treatments showed significantly higher soil nutrient content, grain yield, microbial abundance, and SMF. With the exception of archaeal diversity, the CF treatment exhibited the lowest bacterial, fungal, and protist diversity, and the simplest microbial co-occurrence network. In contrast, the CFR treatment had the lowest archaeal diversity, but the highest bacterial, fungal, and protist diversity. Moreover, the CFR treatment exhibited the most complex microbial co-occurrence network with the highest number of nodes, edges, and interkingdom edges. These results highlight that both chemical fertilization with and without straw retention caused high ecosystem multifunctionality while changing microbial association oppositely. Furthermore, these results indicate that rice straw retention contributes to the development of the soil microbiome and ensures the sustainability of high-level ecosystem multifunctionality.

Long-term partial replacement of mineral fertilizer with in situ crop residues ensures continued rice yields and soil fertility: A case study of a 27-year field experiment in subtropical China.

High yields and environment-related issues because of over-fertilization in rice (Oryza sativa L.) production is a major concern in China. Partial replacement of mineral fertilizer (MF) with organic matter is considered a win-win approach for resource-saving and environmentally friendly rice production. Here, we examined the effects of reduced MF and in situ crop residue on the rice yield and soil fertility in the long term. A 27-year field experiment (a randomized block design with three replicates) in subtropical China was conducted to test the feasibility of the substitution in a double rice paddy ecosystem. The treatments were CT (no fertilizer application considered as control), NPK (mineral fertilizer N, P, and K), and RFC (reduced MF and in situ crop residue to supplement the reduced NPK dose). The crop residue included half of the rice straw and green manure contents, which were retained in situ in the RFC treatment. The RFC maintained the same rice yield and soil fertility levels as NPK. In general, soil organic carbon (SOC) and total nitrogen (TN) content in RFC increased by 10.3% -20.8%, and 7.5% -28.0%, respectively, than that in NPK from the 5th to the 25th years. There was no significant difference in the content and net accumulation of SOC, TN, and TP and soil available nutrients between the RFC and NPK treatments after 25 years. The average annual yields were 9690 and 9872 kg ha-1 for the NPK and RFC treatments, respectively. There was no difference in the yield of the first, second, and annual rice crops between RFC and NPK in most years (six of the fifty-four seasons showed a significant difference). RFC increased the partial factor productivity (PFP), agronomic efficiency (AE) of MF, and yield stability (CV) (p < 0.05). Positive nutrient balance and a reduced loss of nutrients are evident reasons for achieving better indicators (PFP, AE, and CV) for nutrient compensation and organic nutrient utilization in the RFC treatment. The partial replacement of MF with in situ crop residue retention, is a simple and efficient way to maintain the soil fertility and rice yield as NPK in southern China.

Warming Shapes nirS- and nosZ-Type Denitrifier Communities and Stimulates N2O Emission in Acidic Paddy Soil.

Warming strongly stimulates soil nitrous oxide (N2O) emission, contributing to the global warming trend. Submerged paddy soils exhibit huge N2O emission potential; however, the N2O emission pathway and underlying mechanisms for warming are not clearly understood. We conducted an incubation experiment using 15N to investigate the dynamics of N2O emission at controlled temperatures (5, 15, 25, and 35°C) in 125% water-filled pore space. The community structures of nitrifiers and denitrifiers were determined via high-throughput sequencing of functional genes. Our results showed that elevated temperature sharply enhanced soil N2O emission from submerged paddy soil. Denitrification was the main contributor, accounting for more than 90% of total N2O emission at all treatment temperatures. N2O flux was coordinatively regulated by nirK-, nirS-, and nosZ-containing denitrifiers but not ammonia-oxidizing archaea or ammonia-oxidizing bacteria. The nirS-containing denitrifiers were more sensitive to temperature shifts, especially at a lower temperature range (5 to 25°C), and showed a stronger correlation with N2O flux than that of nirK-containing denitrifiers. In contrast, nosZ-containing denitrifiers exhibited substantial variation at higher temperatures (15 to 35°C), thereby playing an important role in N2O consumption. Certain taxa of nirS- and nosZ-containing denitrifiers regulated N2O flux, including nirS-containing denitrifiers affiliated with Rhodanobacter and Cupriavidus as well as nosZ-containing denitrifiers affiliated with Azoarcus and Azospirillum. Together, these findings suggest that elevated temperature can significantly increase N2O emission from denitrification in submerged paddy soils by shifting the overall community structures and enriching some indigenous taxa of nirS- and nosZ-containing denitrifiers. IMPORTANCE The interdependence between global warming and greenhouse gas N2O has always been the hot spot. However, information on factors contributing to N2O and temperature-dependent community structure changes is scarce. This study demonstrated high-temperature-induced N2O emission from submerged paddy soils, mainly via stimulating denitrification. Further, we speculate that key functional denitrifiers drive N2O emission. This study showed that denitrifiers were more sensitive to temperature rise than nitrifiers, and the temperature sensitivity differed among denitrifier communities. N2O-consuming denitrifiers (nosZ-containing denitrifiers) were more sensitive at a higher temperature range than N2O-producing denitrifiers (nirS-containing denitrifiers). This study's findings help predict N2O fluxes under different degrees of warming and develop strategies to mitigate N2O emissions from paddy fields based on microbial community regulation.

Mitigating effects of ex situ application of rice straw on CH4 and N2O emissions from paddy-upland coexisting system.

The in situ application of rice straw enhances CH4 emissions by a large margin. The ex situ application of rice straw in uplands, however, may mitigate total global warming potential (GWP) of CH4 and N2O emissions from paddy-upland coexisting systems. To evaluate the efficiency of this practice, two field trials were conducted in rice-rice-fallow and maize-rape cropping systems, respectively. Year-round measurements of CH4 and N2O emissions were conducted to evaluate the system-scaled GWP. The results showed that CH4 accounted for more than 98% of GWP in paddy. Straw removal from paddy decreased 44.7% (302.1 kg ha-1 yr-1) of CH4 emissions and 51.2% (0.31 kg ha-1 yr-1) of N2O emissions, thus decreased 44.8% (7693 kg CO2-eqv ha-1 yr-1) of annual GWP. N2O accounted for almost 100% of GWP in upland. Straw application in upland had insignificant effects on CH4 and N2O emissions, which increased GWP only by 91 kg CO2-eqv ha-1 yr-1. So, the transfer of straw from paddy to upland could decrease GWP by 7602 kg CO2-eqv ha-1 yr-1. Moreover, straw retention during late rice season contributed to 88.2% of annual GWP increment. It is recommended to transfer early rice straw to upland considering GWP mitigation, nutrient recycling and labor cost.

Estimation of long-term Ca(2+) loss through outlet flow from an agricultural watershed and the influencing factors.

Soil Ca(2+) loss from agricultural lands through surface runoff can accelerate soil acidification and render soil degradation, but the characteristics of Ca(2+) loss and influencing factors in watershed scale are unclear. This study was carried out in a watershed with various land uses in a subtropical region of China. The outlet flow was automatically monitored every 5 min all year round, and the water samples were collected twice a year from 2001 to 2011. The concentrations of Ca(2+), Mg(2+), K(+), total nitrogen (TN), and total phosphorus (TP) of water samples were measured. The dynamic losses of the nutrients through the outlet flow were estimated, and the relationships between the nutrient losses and rainfall intensity as well as antecedent soil moisture were investigated. The results showed that great variations of nutrient concentrations and losses appeared during the investigation period. The average concentrations of Ca(2+), Mg(2+), K(+), TN, and TP were 0.43, 0.08, 0.10, 0.19, and 0.003 mmol L(-1), respectively. The average Ca(2+) loss reached 1493.79 mol ha(-1) year(-1) and was several times higher than for Mg(2+), K(+), and TN, about 140 times higher than for TP. Rainfall intensity had remarkable effects on Ca(2+) concentration (P < 0.01) and loss (P < 0.05) when it reached rainstorm level (50 mm day(-1)), while a quadratic relationship was observed between antecedent soil moisture and Ca(2+) concentration only when rainfall intensity was less than 50 mm day(-1). In a word, much greater amounts of Ca(2+) were lost from the watershed, and this may be one important contributor to the increasing acidification of acidic soils in subtropical regions.

[Effect of short-time drought process on denitrifying bacteria abundance and N2O emission in paddy soil].

In order to investigate the impact of drying process on greenhouse gas emissions and denitrifying microorganisms in paddy soil, wetting-drying process was simulated in laboratory conditions. N2O flux, redox potential (Eh) were monitored and narG- and nosZ-containing denitrifiers abundances were determined by real-time PCR. N2O emission was significantly increased only 4 h after drying process began, and it was more than 6 times of continuous flooding (CF) at 24 h. In addition, narG and nosZ gene abundances were increased rapidly with the drying process, and N2O emission flux was significantly correlated with narG gene abundance (P < 0.01). Our results indicated that the narG-containing deniteifiers were the main driving microorganisms which caused the N2O emission in the short-time drought process in paddy soil.

[Effects of different nitrogen, phosphorous, and potassium fertilization modes on carbon- and nitrogen accumulation and allocation in rice plant].

Based on a 20-year field site-specific fertilization experiment in Taoyuan Experimental Station of Agriculture Ecosystems under Chinese Ecosystem Research Network (CERN), this paper studied the effects of different fertilization modes of N, P, and K on the accumulation and allocation of C and N in rice plant. The fertilization mode N-only showed the highest C and N contents (433 g kg(-1) and 18.9 g kg(-1), respectively) in rice grain, whereas the modes balanced fertilization of chemical N, P and K (NPK) and its combination with organic mature recycling (NPKC) showed the highest storage of C and N in rice plant. In treatments NPK and NPKC, the C storage in rice grain and in stem and leaf was 1960 kg hm(-2) and 2015 kg hm(-2), and 2002 kg hm(-2) and 2048 kg hm(-2), and the N storage in rice grain was 80.5 kg hm(-2) and 80.6 kg hm(-2), respectively. Treatment NPK had the highest N storage (59.3 kg hm(-2)) in stem and leaf. Balanced fertilization of chemical N, P, and K combined with organic manure recycling increased the accumulation of C and N in rice plant significantly. Comparing with applying N only, balanced fertilization of chemical N, P, and K was more favorable to the accumulation and allocation of C and N in rice plant during its growth period.

[Effect of long-term fertilization on soil plough layer carbon storage in a reddish paddy soil].

With a reddish paddy soil as test materials, soil profile nitrogen storage in long-term different fertilization system (1990-2006) have been investigated. The result indicated that recycling of organic matter significantly increased the soil profile C storage (ranged from 57.7 to 66.2 t/hm2), and it was increased by 18.7-27.2 t/hm2 compared with the soil profile C storage of 1990. But it was increased by 5.4 t/hm2 with only application of chemical fertilizers. Saturated carbon storage of paddy soils was 84.0 t/hm2, and the C sequestration potential ranged from 17.8-43.7 t/hm2 compared with the current soil profile carbon storage. The result showed that there was a significant relationship between soil bulk density and depth changes of profile soil. The organic C storage would be greatly underestimated by 20.6% or 11.3% if we only take 20 cm or 23 cm as the standard depth in the estimating method for organic C storage, it also would be underestimated the difference of treatments. The combined application of chemical fertilizer and organic matter is optimal for agricultural field based onsoil organic C storage and the carbon sequestration potential.

[Responses of microbial biomass P to the changes of organic C and P in paddy soils under different fertilization systems].

Based on a fifteen years field experiment in double rice-cropping region of subtropical China, the responses of microbial biomass P (MB-P) to organic C and P in red paddy soils under different fertilization systems were investigated. The results indicated that a long-term input of organic carbon sources and the increasing soil organic carbon made soil microbial biomass remain at a high level (MB-C > 800 mg x kg(-1)), being a main reason of the increase of MB-P. Under long-term zero chemical P fertilization, there was a significant decrease in soil total P (P < 0.05), but soil organic P increased by 29.3% on average. The inorganic P forms in deficit were mainly Al-P, Fe-P, Ca-P and O-P, with the lowest content of Al-P (only 0.5 mg x kg(-1) on average). The content of soil MB-P under zero chemical P fertilization was much higher than that of Olsen-P. Correlation analysis showed that there was a significant relationship (P < 0.05) between MB-P and Al-P, from which, it was deduced that the utilization of Al-P, Fe-P, Ca-P and O-P by soil microbes could be the key approach of promoting these P forms transformed into available P. Chemical P fertilization combined with organic nutrient recycling could not only enlarge the soil P pool, but also improve the P availability.