Short-term effects of climate smart management practices on sorghum (Sorghum bicolor (L.) Moench) productivity, soil organic carbon fractions and aggregate stability in contrasting agroecological zones in South Africa.

Abstract

Addressing climate change calls for the adoption of climate smart practices including the use of resilient, underutilized crops, to increase yields especially on marginal soils, which are widely used by smallholder farmers, particularly in sub-Saharan Africa. A short-term study was conducted to determine the effects of planting date, tillage system, sorghum (Sorghum bicolor (L.) Moench) cultivar and rotations on crop productivity and selected quality parameters of two contrasting soils in KwaZulu-Natal (KZN) and Free State (FS) Provinces (sand clay loam and sandy loam, respectively) of South Africa. The experiment consisted of two sorghum cultivars (Pan8816 and Macia), sown in December (First planting date) and in January (Second planting date) the three years 2020/21 and 2021/22 and under conventional tillage (CT) and no-till (NT), with sorghum-cowpea (Vigna unguiculate (L.) Walp)-sorghum and sorghum-sorghum-sorghum rotations arranged in a randomized complete block design (RCBD) with a 2×2×2×2 factorial experiment. In KZN, none of the factors, except cultivar in the second season had significant effects on yield and the uptake of nitrogen (N), phosphorus (P), and potassium (K) in either season. Pan8816 had higher yield and uptake of N, P and K than Macia, in the second season. In FS, Pan8816 took up 1.74 times more P than Macia, while early-planted sorghum had significantly higher yield and uptake of N, P and K than when planted late in both seasons. Additionally, sorghum plants grown under CT had significantly higher yield and K uptake than under NT. The main effects of planting date, tillage, season and their interactions did not affect microbial biomass carbon (MBC) in FS, except under CT, where in season one MBC was 3.8 times higher than in season two, when the sorghum was planted late. In KZN, only tillage affected MBC with NT resulting in 7% higher MBC than CT. No treatment effects were observed on β-glucosidase. Phosphomonoesterase activity was higher in season 1 than season two at both sites. Late planting under CT (26.75%) resulted in higher water stable aggregates (WSA) in the >250 μm size class, while NT affected WSA of the size range 53-250 μm and < 53 μm than under CT soils, in FS. In KZN, the interaction between planting date and tillage had significant influence on both > 250 μm and 53 μm – 250 μm aggregate sizes, while the main treatments did not. However, late planting resulted in higher <53 μm fraction compared to early planting. Soils from early-planted sorghum under CT had significantly (p < 0.05) higher WSA in both the 53 μm -250 μm and in the > 250 μm than late planting under CT and NT for both tillage systems. Planting date had a significant effect on the <53 μm aggregate size in both sites. In FS, there were no treatment or interactive effects on SOC and the same was observed for all the carbon fractions in KZN. Late planted soils had 6.3% higher OC than early planted soils in KZN. Fine particulate organic carbon did not respond to planting date, tillage or their interaction at both sites. In FS, late planting resulted in a 24.1 % increase in coarse particulate organic carbon (cPOC) compared to early planting. Soil microbial properties were influenced by interactions among PD, tillage and seasonal characteristics in FS while in KZN, tillage effects were predominant. These findings suggest that these climate smart agricultural practices improve aggregate stability, while other effects are site specific, with greater responses of sorghum productivity in the drier FS area, while soil OC and microbial properties were improved in wetter KZN. In the drier FS, early planting under CT is critical for sorghum productivity, while in the wetter KZN, selecting the appropriate cultivar (e.g. PAN8816), is more essential on sorghum productivity, at least in the short-term. Early planting under NT gives a more metabolically active microbiome in the FS soils while CT and season 2020/21 does the same for soils in KZN. Season 2020/21 also resulted in higher numbers and abundance of microbe species in KZN.