Higher carbon sequestration potential and stability for deep soil compared to surface soil regardless of nitrogen addition in a subtropical forest

Greater influences of nitrogen addition on priming effect in forest subsoil than topsoil regardless of incubation warming

Subsoil (below 20 cm), storing over 50 % of soil organics carbon (SOC) within the 1 m depth, plays a critical role in regulating climate and ecosystem function. However, little was known on the changes in SOC decomposition induced by exogenous C input (i.e., priming effect) across the whole soil profile under nitrogen (N) enrichment and climate warming. We designed an incubation system of soil columns with minor physical disturbance, which allows the manual additions of exogenous C and N and incubation under ambient or elevated temperature. A negative priming effect by glucose was observed in all layers of ambient soil, while the negative priming effect was enhanced by soil depth but inhibited by warming. Nitrogen addition shifted the priming effect from negative to positive under ambient temperature, and decreased the magnitude of negative priming effect under elevated temperature. Nitrogen uplift effect on priming effect was more pronounced in subsoil compared to topsoil, while this effect diminished with rising temperature. Soil microbial activity (e.g., the CO2 production within 3 days) and acid phosphatase activity had important roles in regulating the variations in priming effect across the soil profile. Our results indicated that increase in labile substrate (e.g., exogenous C input) input would not lead to native SOC destabilization in subsoil, N addition shifted the priming effect from negative to positive, increasing the SOC decomposition under ambient temperature, while labile C input together with N addition benefited SOC sequestration by inducing negative priming effects in forest soil under warming climate.

Effects of defoliation and nitrogen on carbon dioxide (CO2) emissions and microbial communities in soils of cherry tree orchards

Background

In farmland, microbes in soils are affected by exogenous carbon, nitrogen, and soil depth and are responsible for soil organic carbon (SOC) mineralization. The cherry industry has been evolving rapidly in northwest China and emerged as a new source of income for local farmers to overcome poverty. Accordingly, it is highly imperative to probe the effect of defoliation and nitrogen addition on carbon dioxide (CO₂) emissions and microbial communities in soils of dryland cherry orchards.

Methods

CO₂ emissions and microbial communities were determined in soil samples at three depths, including 0-10 cm, 10-30 cm, and 30-60 cm, from a 15-year-old rain-fed cherry orchard. The samples were respectively incubated with or without 1% defoliation under three input levels of nitrogen (0 mg kg^-1, 90 mg kg^-1, and 135 mg kg^-1) at 25°C in the dark for 80 days.

Results

Defoliation and nitrogen addition affected CO₂ emissions and microbial communities and increased microbial biomass carbon (MBC), the activity of soil catalase, alkaline phosphatase, and cellulase in soils of the dryland cherry orchard. The culture with defoliation significantly promoted CO₂ emissions in soils at the three depths mainly by increasing the MBC, catalase, alkaline phosphatase, and cellulase activities, resulted in positive priming index. Nitrogen addition elevated the MBC and changed soil enzymes and reduced CO₂ emissions in soils at the three depths. Moreover, the priming index was higher in deep soils than in top and middle soils under the condition of defoliation and nitrogen addition. No significant differences were observed in the soil bacterial diversity (Chao1, Shannon, and Simpson) among all treatments. Meanwhile, the relative abundance of Proteobacteria was markedly increased and that of Acidobacteria was substantially diminished in soils at the three depths by defoliation and nitrogen addition. The results sustained that defoliation and nitrogen can regulate SOC dynamics by directly and indirectly affecting soil microbial activities and communities. As a result, the combination of defoliation return and nitrogen fertilization management is a promising strategy to increase SOC and promote soil quality in dryland cherry orchards.