Introduction Methodology Objective: To determine how plant litter quality impacts the transfer of plant residues to SOM fractions stabilized via organo-mineral association. Results Results & Conclusions Can We Build Soil Health Through Greater Soil Organic Matter Stabilization? Carolina Cordova*, Dan Olk 2 , Johan Six 3 , Michael Castellano 1 , 1 Iowa State University Dept. of Agronomy; 2 USDA-ARS National Lab. for Agriculture and the Environment, Ames, IA ; 3 Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland * Corresponding author: [email protected] Reference Fig. 2 Effect on the amount of Carbon accumulated from different plant litter additions *. Fig. 3 Effect on the amount of Nitrogen accumulated from different plant litter additions at the end of the incubation *. High quality (Low C:N) Low quality (High C:N) Fig. 1 Plant litter quality rank. (*) Just aboveground plant litter used, collected at maturity. The experiment was conducted as a 180 day laboratory incubation in a fully factorial completely randomized design with the following factors: two soil types (Sandy Loam and Silt Loam subsoils with low SOM), four plant litters (alfalfa, corn, oats, soybean), and two nutrient inputs (with and without). Samples were incubated for four 46-day cycles. At the beginning of each cycle, ground plant litter (8.70 g OC kg-1 soil) was added to each sample. At the end of each cycle, any partially decomposed plant litter was removed by air blowing. Table 1. Selected properties of sub-soils Clarion and Fayette before the incubation. Measurements CO 2 flux was frequently measured to calculate cumulative C mineralization with linear interpolation and numerical integration. Plant litter C transfer to stable SOC mineral fine fractions (<53 μm) was measured prior to and at the end of the incubation by isotopic analysis (δ 13 C natural abundance using a two-pool isotopic mixing model) as well as calculating the mass transfer of C and N (i.e., the increase in soil C and N over the four, 46 day incubation cycles). Soil organic matter (SOM) stabilization relies on the quality of plant litter inputs. Plant litter quality (PLQ) may affect both nutrient availability and long-term SOM stabilization through the transfer of C and N to soil fractions that are stabilized against mineralization by organo-mineral association. PLQ classifies plants according with their chemical composition. Plant litters with low C/N ratios and low polyphenol concentrations are considered to high quality; as a result, they are easily metabolized by soil microbes (Fig. 1). Cotrufo, et al. (2013) proposed that high quality plant litters result in faster and greater SOM stabilization via organo-mineral association because high quality litters yield more microbial residues and microbial residues dominate SOM stabilized via organo-mineral association. Soil type, plant litter and nutrient addition significantly affected the amount of C and N stabilized via organo-mineral association. Soil type played an important role in the SOC stabilization of the accumulated C and N. The Sandy Loam soil had higher C and N accumulation in the <53μm fraction than the silt loam. C mineralization (i.e., CO 2 -C production) was greater for high quality than low quality plant litters. Our results are consistent with the concept that high PLQ promotes greater transfer of organic matter to soil fractions that are stabilized against mineralization via organo-mineral association. Cotrufo F., Wallenstein M., Boot C., Denef K., Paul D. (2013), The microbial efficiency-matrix stabilization (MEMS) framework integrates plant litter decomposition with SOM stabilization. Global Change Biology. 19:988-995. Plant Litter * C:N ratio Alfalfa 16.6 Oats 22.5 Corn 78.5 Soybean 88.9 Acknowledgements This project was funded by USDA NIFA, SENESCYT and the William T. Frankenberger Professorship of Soil Science. Sub-Soil group Clay (%) pH (1:1 H2O) Total Carbon Total Nitrogen Total Carbohydrates C:N ratio -------g kg-1 soil------ Sandy Loam (Clarion) 15.70 7.07 4.07 0.47 0.73 8.73 Silt Loam (Fayette) 31.50 5.12 2.87 0.40 0.67 7.29 Fig. 4 Carbon to Nitrogen ratio of material accumulated in the soil fine fraction <53μm *. Fig. 5 Cumulative amount of CO 2 -C produced *. (*) Data are means and SE, n=4. Significant differences between the soils and treatments marked by different letters (p-value < 0.05). Data are from mass transfer method. However, the δ 13 C and mass transfer methods were highly correlated (R = 0.98; y = x*1.12 + -1.04). Variable p-value Soil <.0001 Plant <.0001 Nutrients (Nut.) <.0001 Soil*Plant 0.0072 Soil* Nut. 0.1601 Plant * Nut. 0.0572 Soil *Plant*Nut. 0.1107 Plant litter Corn Soybean Oat Alfalfa g carbon kg -1 soil <53µm 0 2 4 6 8 10 12 14 16 Sandy Loam No nutrients Sandy Loam with nutrients Silt Loam no Nutrients Silt Loam with Nutrients Variable p-value Soil <0.0001 Plant <.0001 Nutrients (Nut.) <0.0001 Soil*Plant <0.0001 Soil* Nut. 0.0247 Plant * Nut. 0.0104 Soil *Plant*Nut. 0.0059 Plant litter Corn Soybean Oat Alfalfa g nitrogen kg -1 soil <53µm 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 Sandy Loam no Nutrients Sandy Loam with Nutrients Silt Loam no Nutrients Silt Loam with Nutrients Variable p-value Soil <.0001 Plant <.0001 Nutrients (Nut.) 0.0334 Soil*Plant <0.0001 Soil* Nut. <0.0001 Plant * Nut. 0.0022 Soil *Plant*Nut. <0.0001 Plant litter Corn Soybean Oat Alfalfa C to N ratio 0 5 10 15 20 25 30 35 Sandy Loam no Nutrients Sandy Loam with Nutrients Silt Loam no Nutrients Silt Loam with Nutrients Variable p-value Soil <0.0001 Plant <0.0001 Nutrients (Nut.) 0.4935 Soil*Plant 0.1504 Soil* Nut. 0.2399 Plant * Nut. 0.8874 Soil *Plant*Nut. 0.0093 Plant litter Corn Soybean Oat Alfalfa g CO2-Carbon kg -1 soil 0 1 2 3 4 Sandy Loam no Nutrients Sandy Loam with Nutrients Silt Loam no Nutrients Silt Loam with Nutrients