Thirteen years of applying maize-derived organic materials and N fertilizers in North China Plain increased microbial CUE and NUE while increasing SOC and TN through necromass inputs.

Improving soil organic carbon (SOC) and total nitrogen (TN) stocks in croplands is crucial to mitigate climate change and ensuring food security. Soil microbes are important engines driving terrestrial biogeochemical cycles. Their C use efficiency (CUE) and N use efficiency (NUE), defined as the proportion of metabolized organic C and N allocated to microbial biomass, is a key regulator controlling the fate of soil C and N. It is assumed that microbial CUE and NUE will increase with higher organic fertilizers application rates, however, any empirical evidence is scare. Microbial necromass is a large and persistent component of SOC and TN especially under croplands. It is still unclear how the simultaneous cycling of C and N in soils would be affected under longer-term organic fertilizer addition. 

Here, we studied soil microbial CUE and NUE simultaneously using ¹⁸O-H₂O tracer approach in a 13-year winter wheat-summer maize cropping rotation field trial  in the North China Plain.Here organic fertilizers (i.e. straw, manure, compost, biogas residue and biochar) are annually  applied with (optimal N input, N_opt) or without N (zero N input, N₀) addition. We found straw, manure, compost, and biogas residue additions increased microbial CUE by 42-80% in N₀ and 40-77% in N_opt, and NUE by 25-65% and 33-127% in both N₀ and N_opt, respectively. Organic matter addition increased the SOC by 36-150% and 31-137% in both N₀ and N_opt, while the TN increased by 21-55% and 21-70% in both N₀ and N_opt, respectively. Additionally, organic material additions increased the total microbial necromass C and N by 50-83%, 60-97% and 38-80%, 52-93% under N₀ and N_opt, which contributed 14-43% and 33-58% to the SOC and TN, respectively. We concluded that these C-induced enhancements in microbial growth and CUE or NUE and necromass accumulation were mainly owing to an increased C availability (Easily oxidizable organic C, EOC), and fungal fast-growth strategists. Composting most effectively facilitated microbial C and N cycling, promoting SOC and TN accumulation in cropland.