Assessment of long-term management impact on soil C dynamics in subtropical grasslands

View the project final report

• 2012 annual report

Commodities

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

Practices

• Crop Production: agroforestry, forestry
• Natural Resources/Environment: carbon sequestration
• Production Systems: agroecosystems
• Soil Management: organic matter, soil analysis

Assessment of long-term soil carbon (C) dynamics under a gradient of management intensity in subtropical grasslands, indicated that tree-grass integrated silvopasture is more beneficial for C sequestration, compared to baseline native rangeland or intensively managed sown pastures. Although the mixed C₃-C₄ composition of silvopasture limited full elucidation of C sources, the loss of relic stable C fraction and the increase in recent (C₄-derived) C in sown pasture suggests that C inputs from the sown grass species (bahiagrass) contributed to stable C sequestration. Data also indicated that conversion from native rangeland to sown pasture elevated C losses through respiration.

Introduction

Terrestrial ecosystems are sensitive to human-induced and nature-driven changes, and this impacts their overall integrity and function. Soil C plays a critical role in grassland sustainability and productivity. Its key functions include i.) serving as a source for plant nutrients, ii.) improvement of water-holding capacity of the soil, iii.) formation and stabilization of soil aggregates, iv.) provision of habitable condition for soil microbial diversity, v.) reduction of greenhouse gases by serving as sink for CO₂ (Weil and Magdoff, 2004). However, C stored in grassland vegetation and soil can change in response to a wide array of management and environmental factors. In Florida, grassland ecosystems cover ~2.5 million ha (~17.5% of total land) and supports 1.7 million cattle and calves (USDA-NASS, 2009). There are growing concerns over current and predicted land-use intensification in Florida due to population-driven demand for increased productivity per unit land area, including grasslands (White et al., 2000; Mulkey, 2007). Hence, management strategies that can enhance soil C sequestration and preserve soil resources in the long-term are essential to ensure ecological balance and sustainability while optimizing land productivity. This is especially important under subtropical conditions like Florida, where the characteristic high moisture, flat topography, and sandy composition of the soils favor rapid decomposition of soil organic matter and leaching of organic compounds (NRCS Websoil Survey, 2013). Limited knowledge exists on the dynamics of soil C under such subtropical conditions in southeastern USA. It is uncertain if intensive management which favors higher above-ground production and animal production will have significant long-term effect on soil C dynamics, including CO₂ efflux, compared to low-input management systems such as native rangelands. 

An understanding of the effect of adopted management system on soil C in this ecosystem is indispensable for further development of management strategies that will preserve ecological function and promote optimal soil C sequestration. Moreover, this can be instrumental for informing management and policy decisions on wider grassland ecosystems.

This research was conducted to 

i. Quantify soil total organic C and particulate organic C under 3 distinct management systems by sampling and analyzing soil samples from each >20 year old fields.

ii. Assess for differences in the contribution of above- and below-ground components under the 3 distinct management systems by quantifying the above- and below-ground biomass.

iii. Determine the effect of management intensification on long-term rate of C loss through respiration (CO₂ efflux).