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.
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.
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 105oC 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 105oC for 48 hours. Samples were divided into two subsamples and stored separately at either 4oC or -20oC. 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).
Grazing land intensification promoted SOC and soil N accumulation at the 0 to 90 cm depth. Native rangeland exhibited the smallest SOC stock (76 Mg C ha-1) compared with silvopasture and sown pasture (100 and 110 Mg C ha-1, respectively). No differences were observed in SOC between silvopasture and sown pasture. Similar to SOC, soil N stocks (0 to 90 cm depth) increased in response to grazing land intensification. Soil N stocks increased from 3.3 Mg N ha-1 in the native rangeland to 5.2 and 5.3 Mg N ha-1 in the silvopasture and sown pasture, respectively. No differences in soil N stocks were observed between silvopasture and sown pasture. Our data corroborate previous studies that suggested that a greater proportion of ecosystem C in grass-dominated ecosystems is associated with SOC (Taylor and Lloyd, 1992; Houghton and Hackler, 2000; Liu et al., 2011c). Although native rangeland had greater ability to accumulate C in plant biomass, it had smaller SOC stocks than sown pasture. Thus, plant biomass may not necessarily be related to ecosystem ability to store C in the soil. Other factors such as chemical composition of plant material as well as microbial activity can also affect the amount and characteristics of C inputs and subsequent SOC accumulation.
Management intensification also increased microbial biomass C, potentially mineralizable C, and β-glucosidase activity, particularly in the top 10 cm soil depth. At the 0 to 10 cm depth, potentially mineralizable C was greater in sown pastures (1.2 mg CO2-C kg-1 d-1) than the other land uses (0.5 and 0.6 mg CO2-C kg-1 d-1 for native rangeland and silvopasture, respectively); however, no treatment effect was observed in the 10 to 20 cm depth. Grazing land management intensification affected β-glucosidase activity and there was land use x season interaction. At the 0 to 10 cm depth, the highest β-glucosidase activity was observed in sown pasture in summer (203 nmol g-1 soil hr-1) followed by silvopasture and native rangeland (101 and 72 nmol g-1 soil hr-1, respectively). At 0 to 10 cm depth in winter, silvopasture and sown pastures showed comparable β-glucosidase activities and both were greater than native rangeland (89 and 98 nmol g-1 soil hr-1 vs. 44 nmol g-1 soil hr-1, respectively). At 10 to 20 cm depth, silvopasture and sown pasture also had greater β-glucosidase activities compared with native rangeland in both seasons.
Educational & Outreach Activities
Xu, S., Silveira, M.L., Inglett, K.S., Sollenberger, L.E., and Gerber, S. 2016. Effect of land-use conversion on ecosystem C stock and distribution in subtropical grazing lands. Plant and Soil, 399:233-245.
Xu, S., Silveira, M.L., Inglett, K.S., Sollenberger, L.E., and Gerber, S. 2016Conversion of native rangelands into cultivated pasturelands in subtropical ecosystems – Impacts on aggregate-associated carbon and nitrogen. Journal of Soil and Water Conservation (In review)
Xu, S., Silveira, M.L. 2016. Carbon distribution and changes in 13C NMR spectroscopy under management intensification (In preparation)
Abstracts at National Meetings
- Silveira, M.L., 2014. Soil carbon dynamics in grazing land ecosystems and the impacts of management on soil carbon sequestration and greenhouse gas emissions. ASA-CSSA-SSSA Annual Meeting, Long Beach, Ca, November 2 – 5, 2014.
- Xu, S.g, Silveira, M.L., Inglett, K.S., Sollenberger, L.E., and Gerber, S. 2014. Soil organic carbon stock and distribution in aggregate size/density pools as affected by grazing land intensification in subtropical ecosystems. ASA-CSSA-SSSA Annual Meeting, Long Beach, Ca, November 2 – 5, 2014.
- Xu, S.g, Silveira, M.L., Inglett, K.S., Sollenberger, L.E., and Gerber, S. 2014. Long-term grazing land management intensification effect on soil C stocks in southern Florida ecosystems. ASA-CSSA-SSSA Annual Meeting, Long Beach, Ca, November 2 – 5, 2014.
- Xu, S.g, Silveira, M.L., Adewopo, J.g, and Inglett, K.S. 2013. Impacts of land use change on ecosystem carbon in subtropical grassland ecosystems. ASA-CSSA-SSSA Annual Meeting, Tampa, OH, November 3 – 6, 2013.
- Xu, S.g, Adewopo, J.B.g, Silveira, M.L., and Inglett, K.S. 2013. Impacts of land-use change on ecosystem carbon in subtropical grassland ecosystems. ASA, Southern Branch Meeting, Orlando, FL, February 3 – 5, 2013.
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.
Areas needing additional study
Further studies examining the characteristics and stability of C associated with increased level of intensification are warranted.