Organic Carbon and Nitrogen in Soil Aggregates and Carbon Footprint after Long-term Bio-cover, Crop Sequence, and Phosphorus Management under No-tillage

The use of agronomic conservation practices can enhance soil C and N sequestration via mitigating C and N emissions. However, the long-term influences of bio-covers, crop sequences, and phosphorus (P) fertilization on aggregate-associated soil organic C (SOC) and N and C-equivalent emissions are largely unknown in no-tillage systems. The objectives of this research were to determine soil aggregate-associated C and N status and C footprint, net C gain, and sustainability index under crop sequences of corn, soybean, and cotton interacted with bio-covers of poultry litter, hairy vetch, winter wheat, and fallow, which represent the major cropping systems in the Mid-south; and examine the effects of P application rates (0, 29, 59, 88, and 117 kg P ha-1) on SOC and N on field with low to medium P fertility levels under corn-wheat-soybean systems. The long-term bio-cover and crop sequence experiments (2002-2017) and P fertilization trials (2009-2018) at Milan and Springfield, Tennessee under no-tillage were used for this research. Poultry litter resulted in higher SOC and N concentrations in microaggregates (0.053–0.25 mm) at 0-5 cm than hairy vetch, wheat, and fallow. Crop sequences involving corn (continuous corn, corn-soybean, and cotton-corn) caused higher SOC levels in microaggregates at 0-5 cm than the other sequences. Annual application of 88 kg P ha-1 accumulated higher SOC and N than 0, 29, and 59 kg P ha-1 while over-application of P at 117 kg ha-1 decreased SOC on low P soil. Out of six cropping sequences, soybean involving systems of continuous soybean, corn-soybean and soybean-cotton had higher C-footprint of 3.09, 3.09 and 2.48 kg CO2 eq. per kg yield, respectively. In conclusion, a corn involving crop sequence integrated with poultry litter as a winter bio-cover outperforms the other crop sequence and bio-cover systems in terms of serving as SOC and N sinks in microaggregates and enhancing soil C and N sequestration. The SOC and N stocks can be increased with appropriate P application on P deficient soils. Integration of continuous corn or cotton-corn with poultry litter reduces C-footprint but increase net C gain and sustainability index relative to the other cropping systems.