ARS | Publication request: THEORETICAL DERIVATION OF NEW STABLE AND NON-ISOTOPIC APPROACHES FOR ASSESSING SOIL ORGANIC C TURNOVER

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 3, 2005
Publication Date: April 1, 2006
Citation: Clay, D.E., Carlson, G.G., Clay, S.A., Reese, C., Liu, Z., Chang, J., Ellsbury, M.M. 2006. Theoretical derivation of new stable and non-isotopic approaches for assessing soil organic c turnover. Agronomy Journal. 98(3):443-450.

Interpretive Summary: Using previously Carbon budget analysis approaches, the landscape dependency of the cropping system could not be directly assessed. Fractionating the soil into components produced inconclusive results, and the approach requiring the maintenance of the numerous reference sites at numerous landscape positions as explained by was not practical. This study provides an alternative approach for assess Soil Organic Carbon (SOC) dynamics in fields containing substantial variability. The non-isotopic approach, using the root to shoot ratios, it was estimated that 6,200 kilograms of Carbon per hectare (2.2 acres) was required for SOC maintenance. Calculated maintenance rates were directly proportional to estimated root to shoot ratios. The modified Carbon isotope methods were developed quantify landscape effects on SOC mineralization. Sensitivity of the modified approach showed that the effect of simulated root to shoot ratios was limited, and therefore had a minimal impact on calculated Carbon budgets. In elevation zones below527.3 meters and above 527.3 meters, 15.3 and 6.7 %, respectively, of the SOC were mineralized. Landscape differences were attributed to: (i) water stress which influenced root and shoot growth characteristics; (ii) runoff that may have transported soil and crop residues from summit/shoulder areas to footslope areas; and (iii) tile-line repairs which may have produced conditions that stimulated SOC mineralization.

Technical Abstract: Techniques for measuring soil organic C turnover in production fields are needed. The objective of this study was to develop new non-isotopic and C-13 stable isotopic techniques for determining SOC turnover and to test the approach on data collected from a 65 ha South Dakota field. The sensitivities of the approaches were tested using a range of simulated root to shoot ratios. In the non-isotopic approach, the model, NHC/SOC (initial) = m (dSOC/dt) + b, where NHC was the average amount of non-harvested C returned to soil, dSOC/dt was the average annual change in SOC, b was ratio between SOC and NHC mineralization rate constants, and m was the inverse of SOC multiplied by k(NHC). Calculated maintenance rates and SOC and NHC mineralization kinetics were influenced by root to shoot ratios. For moderate root to shoot ratios (0.8 for corn and 1.0 for soybean) annual applications of 6,200 kg C per hectare was required to maintain SOC levels. In the isotopic approach, the amount of SOC retained in the soil for the moderate root to shoot ratio was calculated using an equation incorporating the C-13 discrimination values (greek delta) of soil at the end of the experiment, plant material remaining in the soil after mineralization, and the delta value of the initial SOC after mineralization. The isotopic approach determined that 15.3 and 6.7 % of the SOC measure in 1995 (SOC-initial) in elevation zones < 527.3 m and > 527.3 m, respectively were mineralized, and 7,420 and 2,780 kg C/ha of new C was incorporated into surface soil SOC in 523.4- 527.3 and 527.3-529.2 m elevation zones, respectively. Landscape differences were attributed to drainage in the footslope areas which stimulated mineralization. The effect of root to shoot ratio on SOC budgeting was limited to their impact on the 'PCR, and therefore root to shoot ratios had a minimal impact on calculated C budgets. For the non-isotopic approach, accurate estimates of root to shoot ratio were required, while the isotopic approach required accurate estimates of delta-PCE.