Chemical stabilization of soil organic nitrogen by phenolic lignin residues in anaerobic agroecosystems

Abstract

This review summarizes independent reports of yield decreases in several agricultural systems that are associated with repeated cropping under wet or submerged soil conditions. Crop and soil data from most of these agroecosystems have led researchers to attribute yield decreases to a reduction in crop uptake of N mineralized from soil organic matter (SOM). These trends are most evident in several long-term field experiments on continuous lowland rice systems in the Philippines, but similar trends are evident in a continuous rice rotation in Arkansas, USA and with no-till cropping systems in North American regions with cool, wet climatic conditions in Spring. Soil analyses from some of these systems have found an accumulation of phenolic lignin compounds in SOM. Phenolic compounds covalently bind nitrogenous compounds into recalcitrant forms in laboratory conditions and occurrence of this chemical immobilization under field conditions would be consistent with field observations of reduced soil N supply. However, technological shortcomings have precluded its demonstration for naturally formed SOM. Through recent advances in nuclear magnetic resonance spectroscopy, agronomically significant quantities of lignin-bound N were found in a triple-cropped rice soil in the Philippines. A major research challenge is to demonstrate in the anaerobic agroecosystems that these lignin residues bind sufficient quantities of soil N to cause the observed yield decreases. A key objective will be to elucidate the cycling dynamics of lignin-bound N relative to the seasonal pattern of crop N demand. Anaerobic decomposition of crop residues may be the key feature of anaerobic cropping systems that promotes the accumulation of phenolic lignin residues and hence the covalent binding of soil N. Potential mitigation options include improved timing of applied N fertilizer, which has already been shown to reverse yield decreases in tropical rice, and aerobic decomposition of crop residues, which can be accomplished through field drainage or timing of tillage operations. Future research will evaluate whether aerobic decomposition promotes the formation of phenol-depleted SOM and greater in-season N mineralization, even when the soil is otherwise maintained under flooded conditions during the growing season.

Introduction

Soil organic matter (SOM) contains the vast majority of C, N, P and S that are found in soil. Its chemical nature is thought to influence the storage and release of these essential nutrients into plant-available forms. Yet evidence for such an influence is sparse, largely due to the inability to determine the exact chemical nature of SOM (MacCarthy, 2001) or even the bonding environments of SOM-bound nutrients. Recent studies have identified the accumulation of phenolic compounds in the SOM of submerged soils that were intensively cropped to irrigated lowland rice (Oryza sativa L.) in the Philippines (Olk et al., 1996, Olk et al., 1998). Using newly developed analytical techniques to identify the bonding environments of C with N, phenolic lignin residues were shown to have bound covalently with N in a humic acid fraction (Schmidt-Rohr et al., 2004). The resulting chemical stabilization was hypothesized to have contributed to an observed long-term decrease in availability of soil N and an associated decline in rice grain yield. In this review we summarize the concepts gained from this study of continuous cultivation to irrigated lowland rice, and we report independent observations from other agroecosystems in which soil remains anaerobic or partly anaerobic during the year and for which soil data suggest the covalent binding of soil N by lignin residues.