Nonadditive Effects on Decomposition of a Mixture of Rice Straw and Groundnut Stover Applied to a Sandy Soil

Wimonsiri Pingthaisong,Patma Vityakon

doi:10.3390/agronomy11061030

Abstract

Rice straw is an abundant resource, but its use as a sandy soil amendment does not increase soil organic matter (SOM) accumulation. Our study aimed to determine the altered decomposition processes that result from mixing rice straw (RS) (low N, high cellulose) with groundnut stover (GN) (high N) relative to applying these residues singly to a sandy soil to identify the mechanisms underlying decomposition of the mixed residues. A microcosm experiment using the litter bag technique showed synergistic, nonadditive effects (observed < predicted values) of residue mass remaining (31.1% < 40.3% initial) that were concomitant with chemical constituent loss, including C (cellulose, lignin) and N. The nonadditive effects of soil microbiological parameters in response to the applied residues were synergistic (observed > predicted values) for microbial biomass C (MBC) (92.1 > 58.4 mg C kg−1 soil) and antagonistic (observed < predicted values) for microbial metabolic quotient (i.e., the inverse of microbial C use efficiency (CUE)) (0.03 < 0.06 mmol CO2-C • mmol MBC−1 • hr−1) and N mineralization (14.8 < 16.0 mg N kg−1 soil). In the early stage of decomposition (0–14 days), mixed residues increased MBC relative to the single residues, while they decreased N mineralization relative to single GN (p ≤ 0.05). These results indicate an increase in microbial substrate CUE and N use efficiency (NUE) in the mixed residues relative to the single residues. This increased efficiency provides a basis for the synthesis of microbial products that contribute to the formation of the stable SOM pool. The SOM stabilization could bring about the SOM accumulation that is lacking under the single-RS application.

Highlights

Degraded tropical sandy soils have low fertility, which can be partly attributed to their low soil organic matter (SOM) or soil organic carbon (SOC) content
The SOM stabilization could bring about the SOM accumulation that is lacking under the single-rice straw (RS) application
The trend of a higher % remaining N in the mixed residues relative to groundnut stover (GN) alone indicated that the mixed residues deterred N loss compared with the single-GN application

Summary

Introduction

Degraded tropical sandy soils have low fertility, which can be partly attributed to their low soil organic matter (SOM) or soil organic carbon (SOC) content. Various carbonaceous (C) compounds, constituents of these residues, are classified as labile or resistant based on their decomposability These two different types of C compounds differ in the extent to which they can restore soil fertility and have profound implications for SOC accumulation and stabilization [2]. In the current model of SOC accumulation resulting from plant litter decomposition, a more significant role in SOC stabilization is assigned to the labile constituents of litters than their recalcitrant counterparts These labile constituents undergo microbial-mediated decomposition during the early stage of decomposition, which produces microbial products that are stabilized in the soil matrix, while the recalcitrant constituents that are mainly decomposed in the later stage produce particulate organic matter that is physically transferred into the soil matrix [6,7]

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