1. 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; cm; 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 ( ) (g DM m-2 y-1)2
656
498
453
487
603
759
470
518
308
D Average residue C input ( ) (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
Dr. Ir. Karolien Denef
phone:
fax:
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).