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.

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.

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.

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.

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.