Long-Term Grazing Exclusion Improves the Composition and Stability of Soil Organic Matter in Inner Mongolian Grasslands.

Chunyan Wang,Nianpeng He,Li Wang,Jinjing Zhang,Yuliang Lv,Dafeng Hui

doi:10.1371/journal.pone.0128837

Abstract

Alteration of the composition of soil organic matter (SOM) in Inner Mongolian grassland soils associated with the duration of grazing exclusion (GE) has been considered an important index for evaluating the restoring effects of GE practice. By using five plots from a grassland succession series from free grazing to 31-year GE, we measured the content of soil organic carbon (SOC), humic acid carbon (HAC), fulvic acid carbon (FAC), humin carbon (HUC), and humic acid structure to evaluate the changes in SOM composition. The results showed that SOC, HUC, and the ratios of HAC/FAC and HAC/extractable humus carbon (C) increased significantly with prolonged GE duration, and their relationships can be well fitted by positive exponential equations, except for FAC. In contrast, the HAC content increased logarithmically with prolonged GE duration. Long-term GE enhanced the content of SOC and soil humification, which was obvious after more than 10 years of GE. Solid-state 13C nuclear magnetic resonance spectroscopy showed that the ratios of alkyl C/O-alkyl C first decreased, and then remained stable with prolonged GE. Alternately, the ratios of aromaticity and hydrophobicity first increased, and then were maintained at relatively stable levels. Thus, a decade of GE improved the composition and structure of SOM in semiarid grassland soil and made it more stable. These findings provide new evidence to support the positive effects of long-term GE on soil SOC sequestration in the Inner Mongolian grasslands, in view of the improvement of SOM structure and stability.

Highlights

Soil organic matter (SOM) plays important roles in retaining and supplying plant nutrients, and in improving soil aggregation and erodibility [1,2]
There were significant increases in the aboveground biomass, litter, soil organic carbon (SOC), Total soil nitrogen (TN), and Total phosphorus (TP) after grazing exclusion (GE), and these parameter were significantly different between grazing grassland (GE0) and longterm GE grasslands
The content of humic acid carbon (HAC) increased from 0.17 g kg–1 in GE0 to 0.36 g

Summary

Introduction

Soil organic matter (SOM) plays important roles in retaining and supplying plant nutrients, and in improving soil aggregation and erodibility [1,2]. SOM, as the largest carbon (C) pool in terrestrial ecosystems, has been commonly divided into active, slow, and passive C fractions according to the turnover time [3]. Six et al [4] divided SOM into protected or unprotected fractions to explore the underlying mechanisms of decomposition. Grazing Exclusion Improves SOM Composition and Stability overlap in stabilization mechanisms, such that the unprotected pool represents the active fractions and part of the slow pool, and the biochemically protected pool is comparable to the passive pool to some extent. Changes in soil humus are supposed to be the most effective component and represent the stability of soil structure and resistance to erosion [8]. Different components of soil humus have specific contributions towards soil fertility according to their humus composition and chemical structure [9]

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Conclusion