Microbial nitrogen-phosphorus imbalance induced by low- and high carbon-nitrogen ratio straw addition promotes soil CO2 emissions

Straw return is a globally prevalent soil management practice designed to enhance carbon sequestration and improve soil health in agricultural ecosystems. However, the effects of straw input with varying quantities and carbon-nitrogen (C/N) ratios on soil respiration (Rs) as well as the balance of carbon (C), nitrogen (N), and phosphorus (P) in both soil and microorganisms remain unclear. In this study, we examined the impacts of straw C/N ratio (high-C/N ratio maize straw and low-C/N ratio peanut straw) and varying amount of straw incorporation on Rs, soil properties, and extracellular enzyme activities. Rs exhibited a significant increase with the addition of straw, regardless of its C/N ratio. This can be attributed to the enhancement in microbial biomass and soil enzyme activity induced by straw incorporation. Low-C/N ratio straw contributed to a continuous increase in soil organic carbon (SOC). However, the SOC content remained unchanged with the increasing input of high-C/N ratio straw. Low-C/N ratio straw increased the soil C/N and C/P ratio, while high-C/N ratio straw increased the soil N/P. Microbial nutrient limitations, assessed through vector analysis, also showed contrasting effects. Increasing the rate of high-C/N straw application significantly reduced vector angle (VA), easing microbial phosphorus limitation. In contrast, higher rates of low-C/N straw increased vector length (VL) and VA, intensifying microbial carbon and phosphorus limitations. High-C/N straw alleviated microbial phosphorus limitation by altering microbial N/P-driven respiration, while low-C/N straw increased carbon limitation through changes in microbial nutrient dynamics. By integrating Rs with soil stoichiometry and microbial metabolic limitations, this study provides a comprehensive exploration of soil respiration’s response to the return of straw with varying straw C/N ratios and quantities.