Karolien Denef1, Catherine Stewart2, John Brenner3, Keith Paustian4 Does long-term center-pivot irrigation increase soil carbon stocks and aggregation in semi-arid agroecosystems? Karolien Denef1, Catherine Stewart2, John Brenner3, Keith Paustian4 INTRODUCTION FIELD EXPERIMENT Tillage management Imperial, NE Combination of minimum tillage (chisel, field cultivator) and no-tillage In the last 3-4 years, a two-pass system is used for corn planting (i.e. one strip tillage operation and corn planter) Otis, CO No-tillage for all crops (for beans, sometimes disking and field cultivation used) Imperial, NE 2 3 1 5 1200 m 400 m 7 6 NG1 N T5N, R39W, s11 T5N, R39W, s10 T5N, R39W, s26 T5N, R39W, s25 4 T5N, R39W, s15 Field selection Imperial, NE 7 cropped fields (1-7) 1 native grassland (NG1) Otis, CO 3 cropped fields (8-10) 1 native grassland (NG2) Irrigation substantially enhances crop production in semi-arid climates, and subsequent greater residue-C input may substantially increase soil C stocks and improve soil structure. However, broad estimates of soil C accumulation based solely on C input may overestimate C accumulation via irrigation by underestimating CO2 loss through increased decomposition. Very few direct comparisons of C stocks and soil aggregation in dryland versus irrigated semi-arid agroecosystems exist, and focus on the surface 0-30 cm of the soil profile and frequently disregard the soil inorganic carbon pool. This inorganic C pool, mainly present in the form of soil carbonates, may be a very dynamic and potentially manageable C pool in (semi)arid systems. OBJECTIVES: To determine if cropland under center-pivot irrigation can sequester additional C and improve soil aggregation compared to rainfed cropland. To determine the relative distribution of carbon stocks through depths in the organic (SOC) and inorganic (SIC) C pools in long-term (over 30 yrs) center-pivot irrigated and dryland farming systems in two climatic regions of the Central Great Plains: a semi-arid (Imperial, Nebraska) and more arid area (Otis, Colorado) Sampling scheme Pivot 4 quadrants/pivot 3 sites/quadrant 3 samples/site (composite) Corners 2 corners/pivot (random) 3 sites/corner Crop rotations Irrigated pivots Corn-Soybean or Corn-Wheat-Soybean (Imperial) Wheat-Corn-Soybean (Otis) Dryland corners Wheat-Corn-Fallow (Imperial) Wheat-Millet (Otis) Otis, CO 8 9 10 T2N, R50W, s24 T2N, R49W, s19 T2N, R49W, s20 NG2 400 m Soil sampling/analyses Depths: 0-5 cm; 5-20 cm; 20-50 cm; 50-75 cm Analyses: Surface texture: all ‘loam’ except ‘sandy-loam’ for NG2 Total C,N: LECO CHN-1000 analyzer Inorganic C: pressure transducer (Sherrod et al., 2002) SOC: calculated by difference Aggregation: wet-sieving (250 mm, 53 mm) (Elliott, 1986) Fig.1 SOC and SIC field variability at all depths for one sampled pivot area (n=12) RESULTS Soil organic carbon (SOC) Pivot-irrigated fields increased SOC stocks in the surface (0-20 cm) soil layers compared to dryland cultivated fields (Fig.2). But, SOC increase was small in comparison with the additional organic C inputs from stimulated plant productivity under irrigation (Table 2). Irrigation is likely increasing microbial activity and C decomposition, decreasing additional SOC storage compared to dryland systems. Soil inorganic carbon (SIC) The SIC pool was small in the native grass sites, but greatly increased under cultivation (Fig.2, Table 1). Irrigation did not significantly change SIC (Fig.2). Though highly variable (Fig.1), especially at deeper depth, SIC in cultivated sites accounted for 30% (Imperial) to 50% (Otis) of the total C (TC) in the 0-75 cm (Table 1). Total carbon (TC) TC increased under irrigation in 0-20 cm due to increased SOC (Fig.2) No significant TC differences in the 0-75 cm layer (Table 1) due to high SIC variability at depth (Fig.1) Table 1: Total 0-75 cm Imperial, NE Otis, CO SOC (g C m-2) SIC TC Native grass 9423 ± 394 507 ± 443 9930 ± 541 5458 ± 261 194 ± 56 5652 ± 253 Dryland 6637 ± 125b 2755 ± 463a 9392 ± 435a 5698 ± 218a 5703 ± 617a 11401 ± 574a Irrigated 7626 ± 102a 2052 ± 349a 9678 ± 333a 6769 ± 190a 5455 ± 555a 12224 ± 467a Imperial, NE Otis, CO Soil structure Cultivation decreased the amount of water-stable macroaggregates (>250 mm) in 0-5 (Fig.3) and 5-20 cm. Irrigation improved soil structure only in the 0-5 cm soil layer, and to a much larger degree in the more arid, no-tilled sites at Otis, CO (Fig.3). Fig.3 Mean proportions and standard errors of aggregate size fractions (0-5 cm only). No significant differences were found between dryland and irrigated in the 5-20 cm depth. Fig.2 Mean soil organic (SOC) and inorganic (SIC) carbon stocks. Error bars represent standard errors of the mean total carbon (TC) stocks CONCLUSIONS Table 2: Differences (D) Irrigated – Dryland Imperial, NE Otis, CO Field # 1 2 3 41 5 6 7 8 9 10 D Average grain yield (2000-2004) (g DM m-2 y-1)2 656 498 453 487 603 759 470 518 308 D Average residue C input (2000-2004) (g C m-2 y-1)3 255 158 145 223 407 505 299 350 216 D SOC stock (0-20 cm) (g C m-2) 774 1018 610 - 196 335 531 662 849 Long-term center-pivot irrigation increases SOC stocks and macroaggregation in the surface soil layers. SIC is less affected by irrigation management, but greatly increases with depth and cultivation. Large proportions of SIC in cultivated semi-arid lands are most likely due to cultivation-induced changes in soil properties, water regime and carbonate-forming ion concentrations. 1 Field 4: only pivot area sampled; 2 Crop yield data were provided by the farmers for the last 5 years. 3 Empirically derived relationships between grain yield, aboveground biomass, belowground & aboveground biomass ratios (cf. IPCC, 2006) and C contributions from crop residues (40%) were used to quantify average annual residue C inputs. AFFILIATIONS CONTACT ACKNOWLEDGEMENTS 1 Laboratory of Applied Physical Chemistry, Ghent University, Gent, Belgium. 2 Department of Geological Sciences, University of Colorado Boulder, CO, USA. 3 USDA Natural Resources Conservation Service, Lakewood, CO, USA. 4 Department of Soil and Crop Sciences and Natural Resource Ecology Laboratory, Fort Collins, CO, USA. Laboratory of Applied Physical Chemistry - ISOFYS Faculty of Bioscience Engineering (FBE) Ghent University (UGent) Coupure Links, 653, B-9000 Gent, Belgium http://www.isofys.ugent.be Dr. Ir. Karolien Denef phone: +32 9 264 6001 fax: +32 9 264 62 30 Karolien.Denef@UGent.be This work was carried out as part of the Consortium for Agricultural Mitigation of Greenhouse Gases (CASMGS) with funding by CSREES/USDA, and with support from the Imperial Young Farmers through funding from USDA/NRCS. Karolien Denef acknowledges a post-doctoral fellowship from the Research Foundation of Flanders (Belgium) (FWO).