Available at: https://digitalcommons.calpoly.edu/theses/2959
Date of Award
12-2024
Degree Name
MS in Environmental Sciences and Management
Department/Program
Earth and Soil Sciences
College
College of Agriculture, Food, and Environmental Sciences
Advisor
Stewart Wilson
Advisor Department
Earth and Soil Sciences
Advisor College
College of Agriculture, Food, and Environmental Sciences
Abstract
Compost application to rangelands has the potential to sequester carbon (C) and add essential plant nutrients to the soil. The California Department of Food and Agriculture's (CDFA) Healthy Soils Project (HSP) provides financial support to farmers and landowners to implement innovative practices that promote soil health. The CDFA is currently recommending compost application rates of 6-10 tons per acre; however, previous research suggests that degraded soils may require a larger dose of compost to overcome limitations, therefore this recommendation might not meet soil health or soil carbon sequestration objectives. This study examines the compost rate effects on soil health, while also utilizing a comparison between soils with differing ages but similar environmental factors through the use of a rangeland chronosequence. Previous studies lack the combination of rate comparisons paired with soil development.
Compost was applied at rates of 0, 10, 20, and 30 tons/acre across the two marine terraces, T1 and T2. Terrace one (T1) is the less developed sandy loam, approximately 50,000 years old, and terrace two (T2) is the more developed sandy clay loam soil, approximately 120,000 years old. The interactions between the treatment and pre-existing mineral soil properties were examined to quantify the mechanisms behind C accrual and soil health on degraded soils. A randomized block design was utilized with 4 blocks per terrace, each 1-acre block containing each of the four treatments (Control, Low, Moderate, High).
Carbon sequestration was measured by testing soil GHG emissions as well as various pools of C within the soil such as total soil carbon (TC), labile soil carbon (POXC), and mineralizable carbon (Min C). Soil health factors were analyzed through measuring soil cations (Mg2+, Ca2+, Na+, K+), micronutrient/heavy metal availability (Znex, Mnex, Feex, Cuex), phosphorous (Olsen P), soil pH, total nitrogen (N) mineralization of nitrogen (PMN). Soil physical properties such as aggregate stability, water holding capacity (WHC), bulk density (Db), and aboveground biomass were also measured. Pre-existing site characteristics such as amorphous iron and aluminum oxides (Fe/Al-oxides) were examined as possible mechanisms for C storage. Data was analyzed via ANOVA and Tukey HSD mean separation to test significance. Linear mixed and mixed effects models were created to evaluate significance of site characteristics and assess which characteristic drives variability in soil C across the terraces.
Year one results show a statistically significant increase in percent carbon (TC) and labile carbon (POXC) in the top 5 cm of T2 between the 30 ton/acre treatment compared to 10 ton/acre treatment and the control. Similarly, levels of extractable Ca2+, K+, and Mg2+ were significantly higher in the plots with 30 t/acre of compost compared to the control plots in T2, as well as for Kex, in T1. Finally, WHC significantly improved in the soil treated with 30 t/acre of compost compared to 10 t/acre in T2, and Db significantly improved in the soil treated with 30 t/acre of compost compared to the control in T2. One-year post-application, soil C, soil mineralogy/health, and soil physical properties significantly increased and/or improved when treated with the high 30 t/acre compost levels compared to the low and/or control treatments.
Two years after compost application, we see similar results with increased TC, Caex, and Mgex in the high application rates compared to the low and control in T2, and we also see the emergence of increased total nitrogen (TN) and extractable Znex in the 30 t/acre treatment plots compared to the 10 t/acre and control plots in T2. However, two years post-application, POXC, WHC, and Db now longer showed significant differences between any rates in T2, though they each appeared to follow the same trend.
Interestingly, results three years post-application showed continued TC and TN, Ca2+ and Mg2+ and Znex significance in the 30 t/acre treated plots compared to the low 10 t/acre plots in T2. Furthermore, TC, Min C, and TN all significantly increased in the 30 t/acre treatment plots in 2023 compared to the level’s measures in the 30 t/acre plots in 2021, suggesting continued C-sequestration and soil health benefits up to three years after a single compost application. Other soil health components such as POXC, extractable cations (K+, Ca2+, Na+, and Mg2+), Znex and Cuex, did not continue to increase into year three, but rather remained constant or slightly decreased, indicating an initial spike in nutrients immediately after compost application, followed by a decline back to “normal” values for some aspects of soil health. However, Alox did emerge as significantly higher under the 30 t/acre treatment compared to the 10 t/acre treatment in T2 in year three and showed a trend towards increasing levels of Alox between 2021 and 2023. This could indicate a lag in the effects of compost on the soil, with some nutrients emerging as significant three years after compost application.
An analysis of correlation between C content (TC, Min C, and POXC) and mineralogical properties (Fe/Al-oxides, clay percent) and the compost treatment levels were determined using linear mixed-effects models (MEM). Across all three years, the linear MEM showed that Feox, Alox, and clay percent were the main predictors of TC storage, with Alox showing the strongest effect, while compost treatment was significant, but had a smaller effect size. For Min C, the MEM shows Alox is the most significant predictor across all years, but an R2 of only 0.18 suggests that factors not included in the model, or year-specific conditions, are also affecting MinC. The MEM for POXC shows that Feox, Alox and clay impact POXC in 2021, with Feox influencing POXC across three years.
No significant treatment effects were observed on amorphous iron (Feox) or clay content on either terrace in any year, meanwhile Alox levels significantly increased with increasing compost application in 2022 and 2023. between terraces in 2022 and 2023, with T2 showing higher levels compared to T1. The linear MEM results indicate that Feox, clay percent, and Alox all significantly predict C storage within the soil after accounting for compost treatments. Since compost directly affects Alox, it is likely that compost and Alox are collinear, and thus compost indirectly influences TC through increasing Alox. In 2021, Feox and clay percent solely predict C storage, by 2023 Alox emerges as the most significant predictor of C storage. This collinearity may be attributed to the release of Alox into the soil from compost treatments. Alox is both a mediator through which compost influences C, and still a valid, strong predictor of TC storage.
The findings from this analysis indicate 1) an application rate of 30 tons/acre is more effective at sequestering C and improving soil health over the traditional 10 tons/acre, 2) pre-existing site characteristics and differences in pedogenic soil development (Alox and Feox and clay percent) are the key factors modulating soil health outcomes from compost application, and 3) continued C benefits across multiple years may be realized after a one-time application.