Soil biochemical parameters in the rhizosphere contribute more to changes in soil respiration and its components than those in the bulk soil under nitrogen application in croplands

Abstract

Soil respiration (RS), which is the second largest carbon flux between the atmosphere and terrestrial ecosystems, has significant impact on atmospheric CO2 concentration and climatic dynamics. Nitrogen (N) fertilizer has been heavily applied in agroecosystems at the global scale for high crop yields, and plays a major role in regulating RS. Although the respective response of soil biochemical property and RS to N addition has been widely studied, the contributions of soil biochemical parameters especially in the rhizosphere to changes in RS and its components (soil heterotrophic (RH) and autotrophic (RA) respiration) under N application remain poorly understood. The present study aimed to examine whether the rhizosphere effect alters the relationship between soil biochemical properties and RS under N addition in croplands. We conducted N application experiment in a wheat-maize rotation system in the North China Plain. N fertilizer was applied at four different levels during both wheat and maize growing seasons: 0, 120, 180 and 240 kg N ha−1. Soil biochemical parameters (e.g. soil enzyme activities, available N, and glomalin contents), RS and its components were measured under all N treatments. First, N addition only significantly enhanced RA in 2014 (the fifth year of N application) but increased both RA and RH in 2015 (the sixth year of N application) because RH had lower N sensitivity than RA or lower soil moisture in 2014 which weakened the effect of N on RH. Second, soil enzyme activities, glomalin and available N contents in both the rhizosphere and bulk soil were significantly improved by N addition. Third, soil biochemical parameters in the rhizosphere explained the changes in RS and its components than those in the bulk soil. Specifically, soil enzyme activities in the bulk soil were more related to RS and its components than those in the rhizosphere, however, available N and glomalin contents showed contrary results. Our study indicated that the contributions of soil biochemical parameters to RS and its components under nitrogen application vary between the bulk and rhizosphere soil, which should be considered in Earth system models to improve their predictive abilities.