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

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

Summary

Soil organic C (SOC) represents an important carbon (C) pool in terrestrial ecosystem and plays a major role on ecosystem sustainability. However, SOC stocks can change in response land use intensification. The impacts of land use management on SOC have been documented extensively across different natural and managed ecosystems in temperate or tropical regions but rarely in subtropical ecosystems. Grazing land ecosystems are of economic and ecological importance in southeastern USA; thus, it is necessary to understand how grazing land management (i.e., fertilization, grazing management) affects SOC responses. The objective of this study was to investigate the long-term effects of grazing land intensification on SOC stocks and characteristics and microbial community responses in subtropical conditions. The study was conducted in south-central Florida (27^o35’N, 81^o55’W) and was focused on three ecosystems that represent three different levels of management intensification: native rangelands (less intensive system), pine (Pinus elliottii) - bahiagrass (Paspalum notatum) silvopasture systems (moderate intensification), and bahiagrass pastures (intensively managed system). 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. Our hypothesis was that adequate management practices, such as introduction of highly productive C₄ grass, rotational grazing, and N fertilization promote organic matter input and, subsequently, increase SOC accumulation. We also expected that management would affect microbial community structural and functional parameters (microbial biomass, phospholipid fatty acid, microbial activity, and enzyme activity). Results demonstrated that 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 microbial biomass and β-Glucosidase activity increased with grazing land intensification.. 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.

Objectives/Performance Targets

In this study, we evaluated the long-term impacts of grazing land intensification (conversion of extensively managed native rangelands into more intensively managed silvopasture and sown pastures) on ecosystem C, including total ecosystem C stocks and distribution among the various above- and below-ground pools, SOC stocks and characteristics, and microbial community structure and enzyme activity. 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; and 2) 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.

Accomplishments/Milestones

Five sampling quadrats (20 × 20m) were demarcated along a diagonal transect within each replicate field (2 replicate per management system), and a total of 450 random soil cores (5 samples × 3depths × 5 quadrats × 2 replicate fields × 3 management systems) were collected, processed and analyzed for total C and N concentrations. Fifteen soil cores were collected from each experimental unit for root biomass determination (5 quadrats x 3 cores = 15 samples per replicated unit). One additional undisturbed soil core was collected from a randomly selected location in each quadrat for soil depth interval bulk density determination. Soil samples were air-dried and sieved through a 2-mm screen. For total SOC and N determinations, soil samples were pulverized with ceramic beads and concentrations were determined using a Flash EA 1112 Series elemental analyzer (Thermo Fisher Scientific Inc., Waltham, MA). For bulk density determination, soil samples collected at each depth interval were dried at 105^oC until constant weight. Total ecosystem C stock was calculated as the sum of above- and below-ground, litter C biomass C, and SOC. Root samples were gently washed with deionized water and root separation was performed using two sets of sieves (500 and 250-μm sieve mesh). After separation, root samples were oven-dried at 65?C until constant weight. Dry root subsamples were ground (0.425- mm screen) and a subset was combusted at 550?C for 5 hours to determine ash concentration. A second subset was used to determine total C and N concentrations using a Flash EA 1112 Series elemental analyzer (Thermo Fisher Scientific Inc., Waltham, MA). Root C and N biomass were calculated based on ash-free root weight.

For the microbial analysis, eight random soil cores (2.2 cm diameter) were collected (0 to 10 and 10 to 20 cm depths) from each sampling quadrat and composited within soil depth. Soil sampling occurred in the summer and was repeated in the winter. Immediately after collection, soil samples were placed in plastic bags and stored in a cooler with ice until transported to the lab. Coarse roots and rocks were removed and soil moisture concentration was determined by oven-drying a sub-sample at 105^oC for 48 hours. Samples were divided into two subsamples and stored separately at either 4^oC or -20^oC. Microbial biomass C and N (MBN) concentrations were estimated using the chloroform fumigation-extraction method (Vance et al., 1987). Potential C mineralization rate was estimated using the laboratory incubation method (Zibilske, 1994). Potentially mineralizable N (PMN) was estimated using the anaerobic incubation procedure (White and Reddy, 2000). Activity of β -glucosidase enzyme was measured using a modified fluorometric procedure previously described by Waldrop et al. (2004) and German et al. (2012).

Impacts and Contributions/Outcomes

Conversion of native rangeland into more intensively managed silvopasture and sown pasture systems favored ecosystem and SOC accumulation; however, C associated with more intensively-managed grazing land ecosystems was more susceptible to decomposition. Results suggested that grass-dominated grazing lands subjected to more intensive management have the potential to increase labile C input and subsequently promote microbial biomass and activity. Data also demonstrated that extensively managed native rangeland has a higher relative abundance of fungal while more intensively managed silvopasture and sown pasture had more bacteria in microbial community. Data suggest that introduction of more productive plant species such as conversion of C3-dominated native vegetation into C4 grasses and adoption of proper grazing and fertilization management strategies can be beneficial for enhancing C sequestration in terrestrial ecosystems.

Results from this research have been disseminated at professional meetings (ASA-CSSA-SSSA, held in Long Beach, CA) and extension materials. This research is also expected to benefit producers by providing scientific information of the potential gains and losses of carbon in response to grazing land intensification. In addition, this effort will be important for subsequent economic analysis of the trade-offs between intensive/improved production systems and integrated systems. We will disseminate our findings through extension activities (such as field days) and through the widely-read newsletter of the Florida Cattlemen Association. This ongoing research effort will provide timely information for livestock producers about appropriate management decisions that will favor livestock and forage production as well as enhance their opportunity for participation in future involvement on carbon credits trade. Furthermore, research results will be published in refereed journals for wide public and scientific community access.