Soil carbon sequestration: factors influencing mechanisms, allocation and vulnerability

Doctor of PhilosophyDepartment of AgronomyCharles W. RiceIncreasing atmospheric CO2 concentrations and other greenhouse gases have been linked to global climate change. Soil organic C (SOC) sequestration in both agricultural and native ecosystems is a plausible option to mitigate increasing atmospheric CO2 in the short term. Laboratory and field studies were conducted to (1) understand the influence of soil water content on the temperature response of SOC mineralization (2) investigate burn and nutrient amendment effects on biogeochemical properties of tallgrass prairie and (3) assess perennial and annual plant management practices on biophysical controls on SOC dynamics. The laboratory study was conducted using soils collected from an agricultural field, currently planted to corn (C4 crop), but previously planted to small grain (C3) crops. The changes in cultivated crops resulted in a δ¹³C isotopic signature that was useful in distinguishing older from younger soil derived CO2-C during SOC mineralization. Soils were incubated at 15, 25 and 35 oC, under soil water potentials of -1, -0.03 and -0.01 MPa. Soil water content influenced the effect of temperature on SOC mineralization. The impact of soil water on temperature effect on SOC mineralization was greater under wetter soil conditions. Both young and older SOC were temperature sensitive, but SOC loss depended on the magnitude of temperature change, soil water content and experiment duration. Microbial biomass was reduced with increasing soil water content. The first field experiment investigated burn and nutrient amendment effects on soil OC in a tallgrass prairie ecosystem. The main plots were burned (B) and unburned (UB) tallgrass prairie and split plots were nutrient amendments (N, P or N+P including controls). Vegetation was significantly altered by burning and nutrient amendment. Treatment effects on either TN or SOC were depth-specific with no impact at the cumulative 0-30 cm depth. The P amendment increased microbial biomass at 0-5 cm which was higher in unburned than burned. However, at 5-15 cm depth N amendment increased microbial biomass which was higher in burned than unburned. In conclusion, soil OC in both burned and unburned tallgrass prairie may have a similar trajectory however; the belowground dynamics of the burned and unburned tallgrass prairie are apparently different. Another field experiment assessed SOC dynamics under perennial and annual plant management practices. The main plots were grain sorghum (Sorghum bicolor) planted in no-tillage (NT) or continuous tillage (CT), and replanted native prairie grass, (Andropogon gerardii) (RP). The spit plots were phosphorus (+P) and control without P (-P). The P amendment was used to repress arbuscular mycorrhizal fungi (AMF), known to influence soil aggregation. The macroaggregate >250 µm, SOC and TN were higher in RP and NT than CT. The relative abundances of AMF and saprophytic fungi were greater with less soil disturbance in RP and NT than in CT. Therefore, less soil disturbance in RP and NT increased AMF and fungal biomasses. The higher relative abundances of AMF and fungi with less soil disturbance increased macroaggregate formation in RP and NT, which resulted in higher SOC sequestration in RP and NT than CT

Comparative Advantage of Mucuna and Tithonia Residue Mulches for Improving Tropical Soil Fertility and Tomato Productivity

Aims: To evaluate the suitability of Mucuna cochinchinensis and Tithonia diversifolia residue mulches for improving tropical soil fertility and tomato productivity, by determining the residue quality and their effect on specific soil properties and crop yield. Methodology: Experimental plots were treated with inorganic and organic inputs (i.e. comprised a control with no input, mineral NPK fertilizer, residues of Mucuna and Tithonia, and mixture of Mucuna and Tithonia). Results: Soil available P increased from 81.3 to 148.3 mg/kg across treatments, with the highest for mineral fertilizer that differed from the plant residues and control, followed by the plant residues that differed from control (Tukey’s HSD, P = .05). Soil exchangeable K increased from 1.3 to 1.9 cmol/kg across treatments, with the highest recorded for plant residues and mineral fertilizer compared to the control (Tukey’s HSD, P = .05), and correlated with treatments (r = 0.51, P = .05). Soil organic C increased from 2.3 to 2.7% across treatments, with the highest recorded for plant residues compared to mineral fertilizer and control (Tukey’s HSD, P = .05), and positively correlated with treatments (r = 0.75, P = .05). Soil pH increased from 4.7 to 5.8, with the highest for mineral fertilizer that differed from the control (Tukey’s HSD, P = .05), and correlated with the soil available P (r = 0.72, P = .05). Tomato yield increased from 9.5 to 13.5 t ha-1 with the highest recorded for sole Tithonia and Mucuna+Tithonia, followed by sole Mucuna and mineral fertilizer as compared to the control, and correlated with soil organic C (r = 0.71, P = .05) and exchangeable K (r = 0.67, P = .05). Conclusion: Mucuna and Tithonia residue mulches are sustainable organic alternatives to improve tropical soil fertility, either singly or in combination, but Tithonia residue has a better impact on tomato productivity due to the higher content of exchangeable K.

Potential of Dual-Purpose Organic Amendment for Enhancing Tomato (Lycopersicon esculentum M.) Performance and Mitigating Seedling Damage by Mole Cricket (Gryllotalpa africana spp.)

Aim: Efficacy of locally produced dual-purpose organic amendment for improving tomato protection and yield was compared with synthetic pesticides and fertilizers. Methodology: The experiment was setup as randomized complete block with three treatments (control, inorganic and organic) each replicated four times. Results: Treatment was negatively correlated with tomato seedling damage by mole cricket (r = −0.86), with 100% efficacy in the organic treatment compared to 90% in the inorganic treatment and 80% in the control (P = .05). Treatment was negatively correlated with tomato blight (r = −0.57), with 100% blight infestation in the control compared to 8% in the inorganic treatment and 25% in the organic treatment (P = .05). No tomato plant was damaged in the organic treatment, compared to 12.5% in the inorganic treatment and 29.1% in the control (P = .001). The total plant damage was negatively correlated with treatment (r = −0.97) and positively correlated with seedling damage (r = 0.90), blight (r = 0.57) and wilt (r = 0.97). The highest tomato yield occurred in the inorganic treatment with 43.9 t ha-1 and organic treatment with 38.1 t ha-1, which differed (P = .05) significantly from the control with 1.5 t ha-1. Tomato yield correlated positively with the number of leaves per plant (r = 0.66), but was negatively correlated with blight (r = −0.70) and wilt (r = −0.60). The highest number of leaves per plant was recorded in inorganic treatment with 30 and organic treatment with 28, compared to 15 in the control (P = .05). Treatment was positively correlated with number leaves per plant (r = 0.63), while the number of leaves was negatively correlated with blight incidence (r = −0.92). Conclusion: The dual-purpose organic amendment is an effective sustainable alternative for improving tomato protection and yield compared to inorganic inputs