Conservation agriculture effects on soil quality and greenhouse gas emission in a sweet sorghum based cropping system in South Africa

Abstract

The general aim of this study was to assess the potential benefits of CA on marginal soils of South Africa for sustainable sweet sorghum production by determining the effect of CA on soil quality indices and soil CO2 emission in a sweet sorghum based cropping system. Specific objectives were to determine: i) the effect of tillage, rotation and residue management in a sweet sorghum cropping system on aggregate stability, binding agents and the resulting aggregate microstructure, ii) the effect of tillage, rotation and residue management on MBC and activities of β-glucosidase, Urease enzyme and phosphatase in sweet sorghum based cropping system in poor soils in South Africa, iii) the effect of tillage, rotation and residue management on steady state infiltration rate (SSIR), bulk density (BD), penetrometer resistance (PR), field capacity (FC), permanent wilting point (PWP) and total available water (TAW) under sweet sorghum based cropping system in marginal soils in South Africa, iv) the effect of tillage, crop residue and crop rotation management on soil organic carbon and labile carbon pools dynamics in sweet sorghum based cropping system, v) the effect of tillage, rotation and residue management on nutrient availability in bioenergy sweet sorghum based cropping system in marginal soil of South Africa, and vi) soil CO2 emission as influence by tillage, crop rotation and residue retention treatments in a sweet sorghum based cropping system in South Africa. Two tillage levels; no-till (NT) and conventional tillage (CT), two crop rotations i.e. sweet sorghum-grazing vetch-sweet sorghum (S-V-S) and sweet Sorghum-fallow sweet sorghum (S-F-S) and three crop residue retention levels i.e. 0%, 15% and 30% were tested. Aggregate stability, soil organic carbon (SOC) and glomalin related soil protein content (GRSP) were statistically higher under NT than in CT. GRSP was also enhanced by 30% residue retention compared with other residue management practices. NT + S-V-S + 30% treatment combination generally had higher total porosity, pore-size distribution and denser microstructure visualisation. Microbial biomass carbon (MBC) and enzyme activities were higher under NT and 30% retention compared to CT and residue removal. NT+ S-V-S + 30% interaction had the highest MBC content compared to CT + S-F-S + 0%. NT + 30% significantly enhanced β glucosidase activity compared to other rotation residue interaction while S-V-S enhanced acid phosphatase compared to S-F-S. MBC and enzyme activities were positively correlated to each other. SSIR, FC, PWP and TAW were not significantly (P>0.05) influenced by the treatments. Nonetheless, the application of NT + S-V-S + 30% treatment combination had the highest SSIR, FC, PWP and TAW compared to CT + S-F-S + 0%, which is regarded as traditional practice. NT increased BD and PR compared to CT. NT LOC fractions compared to CT, which concurs with previous findings. Cold water extractable organic carbon (CWEOC) and hot water extractable organic carbon (HWEOC), were found to be more sensitive to tillage and strongly positively correlated to SOC. NT + S-V-S + 30% treatment combination had higher MBC compared to other treatment combinations. NT enhanced total nitrogen (TN), total organic nitrogen (TON), magnesium (Mg) and sodium (Na) by 3.19 to 45% compered CT. S-V-S rotation increased ammonium (NH4 +-N) and nitrate (NO3 - -N) by 3.42 to 5.98% compared to SS. 30% residue retention increased NH4 + -N, NO3- -N, available phosphorus (Available P), cation exchange capacity (CEC), calcium (Ca), Mg and potassium (K) by 3.58 to 31.94% compared to residue removal. Application of tillage, crop rotation, residue management and their interaction had no significant (P>0.05) effect on daily and cumulative CO2 fluxes. Despite no statistical difference among treatments, cumulative CO2 flux was 7.88% higher in CT than in NT. Soil temperature was the main factor explaining up to 51% variation in CO2 fluxes and CO2 fluxes were higher during warm-wetter months and cooler-dry months. Application of NT + S-V-S + 30% enhanced soil quality in marginal soils of South Africa. Tillage is the main factor influencing aggregate stability, binding agents and resulting microstructure, followed by residue management under short-term sweet sorghum based cropping. Tillage and residue management are the main components that are vital for the enhancement of MBC, β-glucosidase, acid phosphatase and urease activity in the marginal soils of South Africa. The results also suggest that the different LOC fractions represent different SOC pools, with CWEOC and HWEOC representing pools that seem to be highly sensitive to tillage. Residue management mainly influences soil fertility in South African marginal soils. The application of NT has a potential reduce CO2 emissions compared to CT treatment and CT + S-V-S + 30% treatment combination has potential to increase CO2 fluxes compared to other treatment combinations.