Soil respiration and microbial biomass in a savanna parkland landscape: spatio-temporal variation and environmental controls

Abstract

In southern Text, subtropical woodlands dominated by phics. Na-fixing woody legumes have largely replaced areas that were once grassland. This change in ecosystem structure could modify the biogeochemistry of carbon (C) and nitrogen (N) in this region. The objective of this study was to quantify variation in soil respiration, soil microbial biomass (SMB), and potential C and N mineralization rates in relation to landscape heterogeneity and woody plant encroachment in this region. In addition, the importance of soil water as a control over these biogeochemical processes was evaluated with an irrigation experiment. Annual soil respiration was significantly greater in 2-1 h in remnant grasslands [] wooded landscape elements [] respiration in wooded landscape elements was accompanied by higher SMB and potential C and N mineralization rates compared to grasslands, suggesting the increased soil respiration was partially attributable to enhanced microbial activity. Soil respiration varied seasonally in all landscape elements, with highest rates during months with >30 mm of precipitation. Temporal variation in SMB was less distinct and not synchronous with that of soil respiration, suggesting that seasonal variation in soil respiration was driven by changes in root respiration rather than heterotrophic respiration. Irrigation increased soil respiration but reduced SMB and potential C mineralization rates in all landscape elements, implying that the increase in soil respiration was due to root respiration. Linear combinations of soil temperature, soil water content, and rainfall accounted for approximately 70% of the variation in soil respiration in both irrigated and non-irrigated landscape elements. These relationships were consistent with previous ecosystem-and global-scale analyses indicating soil respiration is controlled primarily by temperature and rainfall, and respiration and nutrient cycling may be secondary to soil abiotic factors at this site. Larger pool sizes and faster turnover rates for soil organic C and total N indicate that soil fertility is enhanced following grassland-to-woodland succession. Efforts to manage grassland-to-woodland succession must recognize that biogeochemical changes inherent in this process may be both a consequence and a direct cause of vegetation dynamics in these ecosystems.