Abstract
Grassland degradation is a worldwide problem that often leads to substantial loss of soil organic matter (SOM). To estimate the potential for carbon (C) accumulation in degraded grassland soils, we first need to understand how SOM content influences the transformation of plant C and its stabilization within the soil matrix. We conducted a greenhouse experiment using C3 soils with six levels of SOM content; we planted the C4 grass Cleistogenes squarrosa or added its litter to the soils to investigate how SOM content regulates the storage of new soil C derived from litter and roots, the decomposition of extant soil C, and the formation of soil aggregates. We found that with the increase in SOM content, microbial biomass carbon (MBC) and the mineralization of litter C increased. Both the litter addition and planted treatments increased the amount of new C inputs to soil. However, the mineralization of extant soil C was significantly accelerated by the presence of living roots but was not affected by litter addition. Accordingly, the soil C content was significantly higher in the litter addition treatments but was not affected by the planted treatments by the end of the experiment. The soil macroaggregate fraction increased with SOM content and was positively related to MBC. Our experiment suggests that as SOM content increases, plant growth and soil microbial activity increase, which allows microbes to process more plant-derived C and promote new soil C formation. Although long-term field experiments are needed to test the robustness of our findings, our greenhouse experiment suggests that the interactions between SOM content and plant C inputs should be considered when evaluating soil C turnover in degraded grasslands.
Highlights
Soil organic matter content is a key indicator of soil health, which determines plant productivity and microbial activity (Magdoff and Weil 2004)
Sequestration capacity of grasslands (Smith 2014). This requires a better understanding of the feedbacks between extant soil organic matter (SOM) content, plant productivity, microbial activity, and aggregate formation, which all contribute to the storage and stabilization of C in grassland soils
Our study highlights the important role of initial SOM content for regulating soil C formation and stability through direct and indirect effects on the turnover of aboveground litter, root C
Summary
Soil organic matter content is a key indicator of soil health, which determines plant productivity and microbial activity (Magdoff and Weil 2004). Grasslands cover 40.5% of the world's land area (Gibson 2009) and many of them are subjected to disturbance from environmental and land use changes, such as over-grazing (Reid and others 2004) and conversion from grassland to cropland (Wright and Wimberly 2013) These disturbances result in large losses of soil organic matter (SOM) and can cause desertification in many grassland ecosystems (Lal 2003; Wang and others 2011; McSherry and Ritchie 2013). There is much debate on the potential of grasslands to act as a sink for carbon (C), it is commonly agreed that better management of degraded grasslands could reduce soil C loss and enhance the C sequestration capacity of grasslands (Smith 2014) This requires a better understanding of the feedbacks between extant SOM content, plant productivity, microbial activity, and aggregate formation, which all contribute to the storage and stabilization of C in grassland soils
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