Soil carbon sequestration potential in semi-arid grasslands in the conservation reserve program

Authors: LI, CHENHUI - Texas Tech University; FULTZ, LISA - Louisiana State University; Moore, Jennifer; Acosta-Martinez, Veronica; HORITA, JUSKE - Texas Tech University; STRAUSS, RICHARD - Texas Tech University; ZAK, JOHN - Texas Tech University; Calderon, Francisco; WEINDORF, DAVID - Texas Tech University

Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/25/2017
Publication Date: 2/15/2017
Citation: Li, C., Fultz, L., Kucera, J.M., Acosta Martinez, V., Horita, J., Strauss, R., Zak, J., Calderon, F., Weindorf, D. 2017. Soil carbon sequestration potential in semi-arid grasslands in the conservation reserve program. Geoderma. 297:80-90.

Interpretive Summary: The Conservation Reserve Program (CRP) has numerous benefits including reduced soil erosion, increased C sequestration, and biodiversity through the conversion of highly erodible cropland to grasslands. The rate and magnitude of these changes varies and the factors that impact these changes are largely unknown in the Southern High Plains (SHP). The SHP has a highly erodible landscape with several challenges for soil conservation efforts as this region has significant acreage under sandy soils (e.g., 51.3 to 84.8 %) with low organic matter content (<1%). Texas leads the nation in number of enrolled acres in CRP, which represents one of the major conservation efforts in the SHP. This study evaluated the impact of increasing years under CRP management on C sequestration on the SHP using a CRP chronosequence that included seven croplands (0 y in CRP), 16 CRP (6-28 y) fields, and three rangelands (benchmark "native" C pool). Regression analysis was conducted to gauge the effects of increasing years under CRP on a suite of C properties normalized by soil texture (% clay+silt concentration), including soil organic C (SOC), particulate organic matter C (POM-C), and microbial biomass C (MBC). The C sources (C3 from previous cropping systems or C4 from CRP grasses) in SOC (SOC-C3 and SOC-C4) and POM-C (POM-C3 and POM-C4) were assessed using stable C isotope signature. Additionally, the role of soil microbes in C sequestration was evaluated by investigating the relationship between MBC and CO2 flux and C sequestration. C sequestration rates of the different C pools were assessed at depths of 0-10 cm and 0-30 cm. The CRP chronosequence data showed that SOC increased at a rate of 0.0837 Mg C ha-1 y-1 and 0.2295 Mg C ha-1 y-1 (based on the average soil clay+silt content of 27 %) at depths of 0-10 cm and 0-30 cm, respectively. Correspondingly, the 0.92 million ha of CRP lands on the SHP had an average C sequestration potential of 76,898 and 210,849 Mg C y-1 at depths of 0-10 cm and 0-30 cm, respectively. This study provided evidence that CRP grasslands with year round organic inputs and ground cover compared to soils under cropland preserved greater microbial biomass, which assist C sequestration from multiple aspects, especially as the precuser of stable SOM. It also showed that the extensive land area enrolled in CRP on the SHP has high potential for C sequestration, but for accurate estimates, C sequestration rates must be adjusted for soil texture, even with similar soil series as in the current study.

Technical Abstract: The Conservation Reserve Program (CRP) in the USA plays a major role in carbon (C) sequestration to help mitigate rising CO2 levels and climate change. The Southern High Plains (SHP) region contains N900.000 ha enrolled in CRP, but a regionally specific C sequestration rate has not been studied, and identification of the C pools and processes important in controlling C sequestration rates remain unresolved. We aimed to address these gaps by utilizing a CRP chronosequence with historical rangeland as a reference ecosystem. Soil samples (0–10 and 10–30 cm) were collected in 2012 and 2014 from a total of 26 fields across seven counties within the SHP and included seven croplands (0 y in CRP), 16 CRP fields that ranged from 6 to 26 y (as of 2012), plus three rangelands. Multiple regression analysis was conducted to gauge the rate of C sequestration under CRP within C pools: soil organic C (SOC), particulate organic matter C (POM-C), and microbial biomass C (MBC), with two additional predictors (soil clay + silt content and precipitation). Despite attempts to control for soil texture by targeting a dominant soil series (Amarillo fine sandy loam), the percent of clay + silt (15.2–48.7%) significantly influenced C accrual. The C sources (C3 from previous cropping systems or C4 from CRP grasses) in SOC and POMC were assessed using stable C isotope signatures. Additionally, the role of soil microbes in C sequestration was evaluated by investigating the relationship between MBC and CO2 flux and C sequestration. SOC increased at a rate of 69.82 and 132.87 kg C ha-1 y-1 and would take approximately 74 and 77 y to reach the rangeland C stocks at 0–10 and 0–30 cm, respectively. The C4-C primarily from the introduced grasses was the main source of C sequestration. SOC gains were essentially due to increases in POM-C and MBC, accounting for 50.04 and 15.64% of SOC sequestration at 0–30 cm, respectively. The highest semi-partial correlation coefficients between the increasing years under CRP restoration and MBC indicated CRP had the strongest effect on MBC compared to other C pools. In addition, increasing soil CO2 flux and MBC:SOC ratio with years of CRP restoration indicated MBC played a critical role in the C sequestration process. Conservation of CRP lands and efforts to sustain perennial systems in this highly erodible landscape should be a high priority of conservation programs. In doing so, significant offsets to increasing atmospheric CO2 levelsmay be achieved in addition to erosion control and improved wildlife habitat.