Quantitative and qualitative herbage yield, and carbon sequestration in subtropical grasslands subjected to different precipitation, grazing management and land use type

Abstract

Rainfall variability in and between seasons, lower soil organic matter content, land degradation, and the associated low biomass yield and poor quality of forage are major constraints to livestock production in the subtropical grasslands of South Africa. In this study, the influences of changes in precipitation, grazing management and other land-use types on carbon (C) and nitrogen (N) sequestration potential, herbage yield and nutritive quality were investigated by collecting data from i) the long-term grazing trial site at Grootfontein Agricultural Development Institute (GADI), an arid grassland in Middleburg, Eastern Cape; ii) various land-use types in a semi-arid area, Pretoria; and iii) short-term experimental plots set up in semi-arid grassland in Pretoria, South Africa. In the first study, various land-use types (Leucaena sp. plots, exclosures, cultivated pasture and croplands) were compared for soil carbon (C) and nitrogen (N) storage. Leucaena spp. plots and exclosured lands improved C and N stocks, due mainly to high biomass returns to the soil, while frequently cultivated pasture and croplands had generally lower C and N stocks. Although soil bulk density and soil chemical parameters such as C, N, calcium (Ca), potassium (K), magnesium (Mg), sodium (Na), and pH were affected by land-use type, it was only the C and N stocks that showed land-use by soil depth interaction. This indicates that C and N are more sensitive to frequent soil disturbances and cultivation, compared with other soil properties, which were less affected by soil depth. In the second study, the long-term impacts of grazing management (namely stocking rate and season of use) on selected soil properties were investigated by collecting soil samples from the long-term grazing experimental site at GADI, Middelburg, Eastern Cape, South Africa. Aristida congesta, A. diffusa and Eragrostis lehmanniana were dominant grass species while Pymaspermum parvifolium, Felicia murcata, Salsola Caluna, Walafrida geniculata, Diospyros austroafricana, Diospyros lycioides were dominant shrub species in the study area (Du Toit et al. 2011). Findings from this study indicated that in this arid region of the country, grazing at contrasting stocking rates (light , 0.78 small stock unt ha-1and heavy, 1.18 small stock unit ha-1) resulted in lower soil organic C and total N concentration compared with the non-grazed control (exclosure). Similarly, both spring and summer grazing resulted in reduced soil C and N stocks, lower water infiltration rate, and higher soil compaction compared with the non-grazed control, due mainly to plant removal from animals feeding on the forage, and animal treading and trampling. Generally, animal exclusion improved C and N stocks and, among others, resulted in higher water infiltration and lower soil compaction. Improved land and livestock management through the establishment of multipurpose tree species and strategic animal exclusion would improve C and N storage in vast potentially restorable grazing areas of South Africa. The last experiment was designed to study the effects of different levels of precipitation (simulated drought) and defoliation intervals on soil-water content (SWC), herbage yield and quality. The study was conducted for two years at Hatfield Experimental Farm, University of Pretoria, using rainout shelters to intercept 0%, 15%, 30% and 60% of ambient rainfall as the main plot treatment and two defoliation intervals (45-day and 60-day) as sub-plot treatments. Both rain interception (RI) treatments and defoliation intervals influenced SWC, herbage yield and quality. Rainfall amount and distribution varied within and between years. This variation was confirmed by the negative climatic water balance (precipitation minus evapotranspiration, PET-ET) in most months during the study period, suggesting that the trial site was moisture stressed mainly during the active plant-growing period owing to high atmospheric water demand. In 2013/14, SWC was reduced in the 60% RI treatment, while there were no differences between RI treatments in 2014/15. Generally, SWC of 40 60 cm was higher, compared with the top (0 20 cm) soil layer, mainly because of high surface evapotranspiration and water uptake by plants in top soil layers. The positive linear relationship between herbage yield and precipitation implies that the amount of precipitation influences grassland biomass production. In this study, the DM yield of dominant species showed differences within seasons as well as between seasons. Generally, grass species contributed a significantly (67%) higher proportion of the total plant biomass and maintained community stability in the sub-climax to climax stages. Forbs and shrubs also played a major role by occupying early niches and protecting soil from wind and water erosion across the RI plots. Thus, maintaining important forbs and shrubs is vital for the revitalization of grassland to restore biodiversity. The simulated severe drought of 60% RI caused detrimental herbage yield decline. However, it resulted in improvements in rain-use efficiency (RUE) and some feed quality attributes (namely crude protein (CP) and in vitro organic matter digestibility (IVOMD). Mid-season (January and February) harvests showed high herbage yield, and the recorded yield was proportional to the amount of rainfall. The study also showed that in dry events, when the rainfall of the site decreased by 30% and 60% of the ambient rainfall, defoliating plants at longer intervals (60-day) improved grassland biomass production. The variations found between and within species in yield and nutritive value under different precipitation and defoliation conditions would possibly help to identify adaptable species in future climate change scenarios and management conditions in moisture-stressed areas of South Africa.