Effect of crop residue quality, nitrogen rate and CNPS stoichiometry on microbial respiration and C pools in a dispersive subsoil

Abstract

Soil organic matter (SOM) is crucial for nutrient cycling, biological health, and aggregate stability. Integrated residue and nutrient management (IRNM) is an innovative method of retaining SOM in cropping systems and involves the incorporation of crop residues with exogenous nutrients at a targeted ratio of 10,000 C:833 N:200 P:143S. The practicalities of IRNM for adoption by farmers remain uncertain as limited data exists on soil C response to residues of varying quality and specific impacts of each nutrient. Furthermore, little is known about the effectiveness of IRNM to build SOM on degraded soils such as dispersive subsoils, and recent studies have found stable SOM can occur in both the mineral-associated organic carbon (MAOC) and particulate organic carbon (POC) fractions. An incubation experiment was conducted to determine the microbial respiration response and changes to mid-infrared spectroscopy derived soil C pools under three residue treatments (no residue (Nil-res), a sorghum residue with low nutrient content (R1) and a sorghum residue with higher nutrient content (R2)), three N rates (Nil N, Half N, Full N), and three nutrient treatments (N, N(+S), and N(+S+P)). The application of residue compared to Nil-res caused an increase in total organic carbon (TOC) from 0.57 % to 0.69 %, MAOC from 0.44 % to 0.51 % and POC from 0.09 % to 0.13 %. Within residue treatment groups, the application of N at the Full N rate was the major determinant of C increase followed by application of N(+S+P) compared to N and N(+S). There was an increase in CO₂ respiration following application of both residues versus Nil-res, and Full N versus Half N and Nil-N. Microbial respiration was significantly higher in the N and N(+S+P) treatments compared to the N(+S) and nutrient control. When residue was applied, the correlation between respiration and C was negative in the nutrient control and positive when applied with N(+S+P). The concomitant increase in CO₂ respiration and MAOC following residue incorporation indicated microbial cycling of C occurred leading to stabilisation. Overall, the study provided initial evidence that microbial cycling of C is mediated by the adequate supply of nutrients and this occurred on a soil that contains abiotic stressors that limit plant growth (therefore C inputs) and microbial health.