Quantitative and Qualitative Responses of Soil Water-Extractable Organic Matter to Carbon and Nitrogen Management Practices in Loess Soil

Yu-long Li,Zhi Qu,Yuan Li,Ming Li,Rong Chai,Qin Chen,Xiao-hong Tian,Zheng-kui Ge,Min Duan,Yan-jiang Zhang,Ai-qing Zhao

doi:10.3390/agronomy11102025

Abstract

Soil-dissolved organic matter (DOM) drives the carbon (C) and nitrogen (N) cycles in agroecosystems. Despite many studies on DOM dynamics, hardly any attention has been directed toward DOM quality, particularly DOM composition. The aim of this study was to elucidate how C and N management practices alter soil water-extractable organic matter (WEOM) in a loess soil agroecosystem. Field experiments were conducted with a winter wheat monoculture. Three N fertilization rates (0, 120, and 240 kg ha−1 year−1) were applied for 17 years (2002–2019), combined with five C practices (zero, low, and high rates of sheep manure or wheat straw) for three years (2016–2019). The results reveal that soil organic carbon (SOC) and water-extractable organic carbon (WEOC) concentrations in the topsoil (0–20 cm) were increased by organic amendments considerably but were not affected by N fertilization. The fluorescence excitation–emission matrix spectra (EEM) of WEOM were resolved to two humic-like components (C1 and C2) and two soluble microbial byproduct-like components (C3 and C4). The proportions of C1 and C2 were increased, while the proportion of C3 was decreased by both C and N management practices. In conclusion, organic amendments increased both WEOM quality and its proportion of humic-like components, whereas N fertilization increased the proportion of humic-like components without variations of WEOM quality in the topsoil of loess soil.

Highlights

IntroductionSoil organic matter (SOM) is critical for both agricultural ecosystems and the global carbon (C) cycle
We found that the Soil organic carbon (SOC) concentrations in the topsoil of a winter wheat/summer fallow system increased by increasing the application rate of wheat straw and sheep manure (Figure 1)
The results of our study indicate that soil water-extractable organic matter (WEOM) composition was altered under C and N management practices, and this is partly due to changes in the succession of the Soil organic matter (SOM) decomposition process because water-extractable organic carbon (WEOC) is in equilibrium with the native soil C [8]

Summary

Introduction

Soil organic matter (SOM) is critical for both agricultural ecosystems and the global carbon (C) cycle. Soil organic carbon (SOC) constitutes ~50% of SOM [4] and represents the largest terrestrial C pool, with an estimated 2400 Pg C up to a soil depth of 2 m globally. The SOC pool is considered to be 2–4-fold the atmospheric pool and 4–8-fold the biotic pool [5,6]. It is essential to investigate the factors that influence SOM quantity and quality, anthropogenic factors in agricultural ecosystems. Studying SOM characteristics—especially molecular properties—remains challenging because physical, chemical, and biological processes all convert dead plant or animal materials into organic compounds that interact with soil minerals [5]

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