Soil organic carbon accretion vs. sequestration using physicochemical fractionation and CQESTR simulation

Authors: Gollany, Hero; FORTUNA, ANN-MARIE - Washington State University; SAMUEL, MARSHALL - Malaysian Agricultural Research And Development; Young, Francis; PAN, WILLIAM - Washington State University; PECHARKO, MIKE - Washington State University

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/8/2013
Publication Date: 3/19/2013
Citation: Gollany, H.T., Fortuna, A., Samuel, M., Young, F.L., Pan, W., Pecharko, M. 2013. Soil organic carbon accretion vs. sequestration using physicochemical fractionation and CQESTR simulation. Soil Science Society of America Journal. 77:618-629.

Interpretive Summary: Accurate estimates of soil organic carbon (SOC) stocks are required when determining changes in SOC resulting from agriculture management practices. A conservation tillage and crop rotation study was established in the summer of 1995, near Ralston in central Adams County in the state of Washington, in a low-precipitation zone (less than 11.8 inch/year) of the eastern Pacific Northwest. Three cropping systems were studied: sweep-tillage winter wheat-tillage fallow rotation, direct seed (no-till) spring wheat-chemical fallow rotation, and direct seed spring barley-spring wheat rotation. Our objectives were: 1) to determine total SOC and estimate the contribution of light fraction carbon (an easily metabolized form of carbon) to total SOC; and 2) to simulate SOC dynamics using the CQESTR carbon model to examine possible effects of climate change for three cropping systems in the Pacific Northwest. Light fraction carbon masked small gains or losses in measured total SOC for all cropping systems. Comparison of the CQESTR model simulation and measured data indicated no significant changes in SOC in the top 12 inches of soil for soft-white winter wheat with sweep tillage fallow and direct seed spring wheat with chemical fallow rotations, whereas SOC increased in the spring barley with hard-red spring wheat crop rotation. Average total SOC over 5 years was 11.48, 11.86 and 12.35 ton/ac for the soft-white winter wheat with sweep tillage fallow, no-till spring wheat with chemical fallow, and spring barley with hard-red spring wheat crop rotation, respectively. The apparent increase (0.87 ton/ac) in SOC with continuous no-till spring cropping was actually accumulated undecomposed crop residues, which we confirmed with light fraction carbon analysis. The average contributions of the light fraction to the total SOC across cropping systems ranged from 13.4 to 18.4% (fall soil samples) and 14.4 to 18.9% (spring soil samples). The CQESTR model predicted no significant change in SOC stocks for either the soft-white winter wheat with sweep tillage fallow or no-till spring wheat with chemical fallow even with a 30% crop biomass increase (based on a potential precipitation increase and crop biomass increase in a climate change scenario for the dryland Pacific Northwest region). Continuous spring cropping under direct seed was predicted to be a more viable management system for SOC accretion than a winter wheat-tillage fallow cropping system for this site under potential climate change scenarios if increased temperature and precipitation improve spring wheat yield and soil organic matter decomposition remains at the current rate. Differences between the observed and predicted SOC stocks were due to artifacts associated with protocols used to determine SOC in a soil that has not reached an equilibrium state following agricultural management change. We confirmed this by light fraction C analysis which provided a first approximation of organic C accretion. Accurate assessment of SOC stock changes requires careful soil sampling and sample processing because it is affected by the depth of sampling, time of year when the samples were taken, as well as the duration since change of management practice. The CQESTR model is a quick, reliable tool to predict passive SOC stocks and is able to overcome artifacts such as crop-residue-derived carbon often associated with total SOC determinations and estimates of soil carbon sequestration potential in small-grain-based cropping systems under dryland climatic conditions. [GRACEnet publication].

Technical Abstract: Accurate estimates of soil organic C (SOC) stocks are required to determine changes in SOC resulting from agricultural management practices. Our objectives were to: (i) determine total SOC; (ii) estimate the contribution of light fraction C (LF-C) to total SOC; and (iii) simulate SOC dynamics using CQESTR to examine the effect of climate change for three cropping systems in the Pacific Northwest. The LF-C masked small gains or losses in measured SOC for all cropping systems. Simulated data indicated no significant changes in SOC in the top 30 cm of the sweep-tillage winter wheat (Triticum aestivum L.)-tillage fallow rotation (WW-TF) and no-till (NT) spring wheat-chemical fallow rotation (SW-CF/NT), whereas SOC increased in the NT spring barley (Hordeum vulgare L.)-spring wheat rotation (SB-SW/NT). The apparent increase in measured SOC with continuous NT spring cropping was the result of accumulated undecomposed crop residues that contributed to the labile C pool and was confirmed via LF-C analysis. The contributions of the LF-C to total SOC across cropping systems ranged from 13.4 to 18.4% (fall soil samples) and 14.4 to 18.9% (spring soil samples). Modeling predicted no significant change in SOC stocks for the WW-TF and SW-CF/NT rotations, even with a 30% crop biomass increase based on potential climate change scenarios. Differences between the observed and predicted SOC were due to artifacts associated with protocols used to determine SOC that did not completely remove accrued crop residue and could be explained by LF-C, which provided a first approximation of organic C accretion. [GRACEnet publication].