Impacts of land use intensification on soil organic carbon stocks, soil carbon fractions and microbial activities in subtropical grazing land ecosystems

View the project final report

• 2014 annual report

Commodities

• Agronomic: grass (misc. perennial)
• Animals: bovine

Practices

• Crop Production: nutrient cycling, nutrient management
• Natural Resources/Environment: carbon sequestration
• Production Systems: agroecosystems

Land use often leads to changes in ecosystem carbon (C) cycling, including major impacts on soil C stocks. Considerable efforts have been placed on understanding land use change impacts on soil C responses in temperate regions; however, much less research has been conducted in subtropical ecosystems. Grazing land management practices such as grazing intensity and nutrient management are expected to affect plant community and soil characteristics; however the direction (either positive or negative effects) and extent that these practices affect ecosystem C responses have not been fully evaluated. In this study, we investigated the impacts of grazing land intensification on ecosystem C and microbial community responses. Treatments consisted of three land use types: native rangeland (less intensively managed), silvopasture and sown pasture (more intensively managed). The impacts of grazing land intensification on above- and below-ground plant biomass, litter biomass, soil organic C (SOC), and soil microbial community structure and activity were evaluated. Silvopasture exhibited the greatest above-ground C biomass (59 Mg ha^-1) compared with native rangeland and sown pasture (4 and 2 Mg ha^-1, respectively). The greatest proportion of ecosystem C was associated with SOC (average of 77% of total ecosystem C). Grazing land intensification promoted SOC accumulation (76 Mg ha^-1 for native rangeland vs. 100 and 110 Mg ha^-1 for silvopasture and sown pasture at 0 to 90 cm depth). However, data also demonstrated that labile C increased with grazing land intensification. Particulate organic C (POC) at 0 to 20 cm soil depth increased from 17 to 28 Mg ha^-1 with the conversion of native rangeland to sown pasture. Similarly, light-free (LF) C fraction also increased in the sown pasture (33 g kg^-1 soil) compared with native rangeland (16 g kg^-1 soil). Microbial biomass and β-Glucosidase activity followed a similar pattern. For instance, average microbial biomass at 0 to 20 cm depth in native rangeland and silvopasture was 213 mg kg^-1 compared with 334 mg kg^-1 in the sown pasture. Data indicated that conversion of native rangelands into more intensively-managed pastures can promote SOC in subtropical ecosystems; however, intensification can also affect soil microbial activity and mineralization of SOC.

Introduction

Despite the high-profile public debate about the impacts of land use intensification on GHG emissions and climate change, our understanding is limited of how grazing land management can be manipulated to promote long-term ecosystem C sequestration. Most previous studies on the impacts of grazing land intensification on ecosystem C responses focused on a single management factor (Fisher et al., 1994; Conant et al., 2001). However, from economic and practical perspectives, grazing land intensification often involves a combination of multiple management practices aimed at increasing productivity; therefore, the interpretation of previous studies focused on single management practices is often limited. Moreover, considering the large area occupied by grazing lands in the USA (~ 30% of the land surface) (Follett et al. 2001), it is critical to understand how grazing land strategies affect SOC and related ecosystem responses. Addressing this knowledge gap is particularly important in the southeast USA where SOC plays a major role in grassland sustainability (Adewopo et al., 2014). Because grassland soils contains appreciable amounts of C and the majority (~90%) of ecosystem C (Burke et al. 1997), relatively small changes in SOC stored in grassland soils can have significant impacts on the global C cycle (Parton 1995).

The overall objective of this research was to investigate the impacts of grazing land intensification on ecosystem C and microbial community responses in a subtropical region. The specific goals of this study were:

1) To determine the long-term effects (> 20 yr) of grazing land intensification on ecosystem C stocks and distribution among the various above- and below-ground pools

2) To quantify and characterize SOC and aggregate size fractions in subtropical grazing lands subjected to different levels of management

3) To evaluate microbial community and process responses to grazing land intensification

The central hypothesis was that management practices intended to increase plant and animal production such as converting native rangelands into silvopasture and sown pastures control production inputs, distribution, and quality and, therefore, have major impacts on ecosystem C dynamics and soil microbial community structure.