Irrigation has a critical role for crop production worldwide. In particular, irrigation is a major issue due to the growing food demand and climate change. Irrigation affects yields and the emission of greenhouse gases such as CO2 and N2O by soils. Here, we review the effect of irrigation on soil organic carbon and N2O emissions. We analysed 22 investigations in various regions of the world. Interactions between irrigation, soil and management factors are described. The main points are: (1) The influence of irrigation is strongly dependent on climate and initial soil organic carbon content. For instance, irrigation of cultivated desert soils led to an average increase of 90 % to over 500 % of soil organic carbon. (2) Irrigation of semiarid regions increases soil organic carbon by 11 % to 35 %. (3) No consistent effects of irrigation were observed in humid regions. In many cases, N2O emissions increase after precipitation or irrigation. (4) Comparison of N2O emissions from irrigated and non-irrigated fields shows that availability of reactive nitrogen compounds controls increased N2O emissions under irrigation, in most cases. Here, increases of about 50 % to 140 % in N2O emissions were reported.

/pdf/irrigation-soil-organic-carbon-and-n2o-emissions-a-review-1ekvwuyyet.pdf

Irrigation, soil organic carbon and N2O emissions. A review

Among global issues of the twenty-first century are: population growth, increase in energy demand, enrichment of atmospheric concentration of greenhouse gases (GHGs) and the attendant climate change, increase in food production needed to meet the demands of growing and increasingly affluent population, and aggravated risks of soil and environmental degradation. Thus, enhancing the magnitude and mean residence time (MRT) of soil carbon (C) pool is integral to any strategy of addressing these and other related global issues. The promise of soil C sequestration is based on the magnitude of net biome productivity (NBP) of ~3 Pg C/year, and the hypothesis that some of the NBP can be retained in the soil to offset emissions and also enhance the resilience of soil and agroecosystems to climate change. Complementary to soil organic C (SOC) is soil inorganic C (SIC) pool, together estimated at ~4,000 Pg to 3-m depth, and play a major role in the global C cycle (GCC). The soil C sink capacity has been enhanced by depletion of the antecedent SOC pool in degraded soils. Despite the large potential of 0.5–1 Mg C/ha/year, there are numerous uncertainties: (1) increased emissions of GHGs with increase in the atmospheric concentration of CO2 because of accelerated decomposition and erosional processes, (2) change in the rate of C uptake in soils of the tropics by climate change, (3) impact of chemical weathering of silicate rocks in altering terrestrial sinks, (4) unknown fate of C transported by erosional processes, (5) a possible positive feedback from the melting of permafrost, (6) decrease in capacity of terrestrial sinks because of soil degradation, and (7) the effects of wild/managed fires on soot, charcoal and NBP.

Soil Carbon Management and Climate Change

Environmental factors controlling soil organic carbon storage in loess soils of a subhumid region, northern Iran

Microbial and enzyme activities are increasingly being recognized as sensitive indicators of soil quality. This study aims to determine the impacts of different kinds of land use/cover, i.e. virgin natural forest (VNF), degraded natural forest (DNF), alder plantation (AP), sequoia plantation (SP), improved fallow (IF) and home garden (HG) areas on soil properties in northern Iran. Five composed samples per land use/cover were taken from the top 10 cm of the soil. Soil microbial respiration was found to be in the ranked order of AP > VNF > HG > DNF > IF ≈ SP areas, being more than one-and-a-half-fold under AP compared with SP. The values of microbial biomass carbon were almost two times higher under SP ≈ VNF than under IF ≈ HG areas. Microbial biomass nitrogen was found to be significantly higher under AP ≈ VNF than in the other kinds of land covers. AP ≈ VNF significantly increased the activities of urease and acid phosphatase, whereas greater activities of invertase occurred in AP ≈ VNF ≈ SP sites. Arylsulphatase activity was significantly higher to be significantly higher under AP in comparison with VNF > HG > DNF > SP > IF areas. As a conclusion, alder (Alnus subcordata C. A. Mey.) plantation improved the soil quality to levels similar to natural forest ecosystems. This research recommends the preservation of natural forests to increase soil quality and a plantation with suitable native broad-leaved species and forestation plan management to rehabilitate degraded natural forests.

Impacts of changes in land use/cover on soil microbial and enzyme activities

Land abandonment might be an alternative management for restoring soil conditions and C from prolonged cultivation and agricultural practices. In the present study, the influence of 18–22 years of land abandonment on soil properties, C dynamics and microbial biomass was evaluated in closely situated wheat and alfalfa farmlands, and abandoned lands on calcareous soils, Central Iran. Soil properties of the 0–15 and 15–30 cm depths from abandoned lands were compared to those from conventionally cultivated lands (i.e., continuous wheat–fallow and alfalfa–wheat rotation) common in calcareous soils of Central Zagros Mountains. Soil bulk density in the 0–15 and 15–30 cm layers decreased significantly while total porosity increased significantly in abandoned lands. Generally, soil aggregate stability tended to increase within the abandoned fields owing to increased water-stable macro-aggregates. Soil organic C (OC) contents (g kg−1) and pools (Mg ha−1) in the 0–15 cm soil layer increased significantly in abandoned lands compared with cultivated lands, with no effect in the 15–30 cm soil layer after 18–22 years of land abandonment, suggesting the restoration of C is pronounced in the upper 0–15 cm soil depth . The total C accumulation in abandoned lands was 7.0 Mg C ha−1 for the entire sampling depth (0–30 cm) over the 18–22 years of land abandonment, which was 26% greater relative to cultivated lands. Carbon mineralization (Cmin) followed a trend similar to organic C, whereas C turnover (Cmin/OC ratio) was slightly greater in wheat fields. However, soil microbial biomass C (MBC) did not vary considerably among the three land uses. In brief, improvements, albeit slowly, in soil properties of the top layer with the cessation of cultivation indicated that land abandonment may result in enhanced soil C sequestration, and would maintain fertility and productivity of the farmlands of semi-arid climates.

Soil properties and C dynamics in abandoned and cultivated farmlands in a semi-arid ecosystem

This study aimed at investigating the effects of agricultural exploitation on desert soil organic C, N and P, and soil aggregation. Four land uses were assessed: (1) 5-year wheat (Triticum aestivum L.)/barley (Hordeum vulgare L.) + 5-year maize (Zea mays L.); (2) 5-year wheat/barley + 5-year alfalfa (Medicago sativa L.); (3) 6-year wheat/barley + 4-year acacia (Robinia pseudoacacia L.) and (4) uncultivated desert soil. The desert soil contained total organic C (TOC) of 3.1, 3.7 and 4.2 g kg-1 and particulate organic C (POC) of 0.6, 0.7 and 0.8 g kg-1 at 0-10, 10-20 and 20-30 cm depths, respectively. The soil TOC concentration was increased by 32-68% under wheat-maize rotation and by 27-136% under wheat-acacia at 0-20 cm depth, and by 48% under wheat-alfalfa only at 0-10 cm depth. This contrasted with an increase in the soil POC concentration by 143-167% at depth 0-20 cm under wheat-maize and by 217%, 550% at depth 0-10 cm under wheat-alfalfa and wheat-acacia, respectively. The desert soil had 13 Mg ha-1 TOC stock and 2 Mg ha-1 POC stock at depth 0-30 cm, whereas crop rotations increased the soil TOC stock by 30-65% and POC stock by 200-350%. Over the 10-year period, the rates of TOC accumulation were 0.6, 0.3, 0.8 Mg ha-1 year-1 and the rates of POC accumulation were 0.4, 0.4 and 0.7 Mg ha-1 year-1 under wheat-maize, wheat-alfalfa and wheat-acacia rotations, respectively. At 0-30 cm depth, total soil N was increased by 61-64% under wheat-maize and wheat-acacia, but total soil P was reduced by 38% under wheat-alfalfa. A significant improvement in clay stability but not in aggregate water-stability was observed in cultivated soils. The results showed a significant increase in soil organic C pool but unimproved macro-aggregation of the desert soil after 10 years of cultivation.

Changes in soil organic carbon, nutrients and aggregation after conversion of native desert soil into irrigated arable land.

This paper investigates effects of cropping abandonment and perennial grass growing on soil organic C and N pools and aggregate stability, by comparing soils under native grassland, crop cultivation, perennial grass growing and cropping abandonment, in degraded cropland at a sub-alpine site in north-western China. The pools of total and particulate organic C (115 and 37 Mg ha-1) in the 0-30 cm soil layer of native grassland were reduced by 31 and 54% after 30 years of crop cultivation. After 4 years of conversion from cropland to perennial grass growing total and particulate organic C pools were increased by 29 and 56%, whereas 4 year cropping abandonment increased particulate organic C by 36%. Rapid increases in total and particulate N were also found in perennial grass growing and cropping abandonment soils. The native grassland soil and soils of cropping abandonment and perennial grass growing had higher carbohydrate C concentrations in the 0-10 cm layer than the cropped soil. The rapid recovery of particulate organic fraction and carbohydrates in the re-vegetated soils were probably due to higher plant biomass inputs and lower organic matter decomposition compared with those in the cropped soil. Aggregate stability of the 0-30 cm soil layer was significantly decreased by crop cultivation but showed a good recovery after 4 year re-vegetations. This study suggests that reduction of soil organic matter and aggregate stability under crop cultivation may be remedied by cropping abandonment or perennial grass growing.

Soil organic carbon and nitrogen fractions and water-stable aggregation as affected by cropping and grassland reclamation in an arid sub-alpine soil

Ethiopia has a large population of small ruminants (sheep and goats) which are mostly kept in traditional subsistence production systems that are poorly described. Understanding these different systems, their population structure, biomass, production, and economic value is essential for further analysis and effective policy making. The objective of this study was to quantify these parameters for small ruminant production systems in Ethiopia to use them as a basis for analysis of disease burden within the Global Burden of Animal Diseases program. Population structure and trends of small ruminants were analyzed using data from ten annual national agriculture surveys. A stochastic herd model was used to simulate the small ruminant population, biomass, and economic value. The model was parameterised stochastically using data from statistical databases and the literature, and sensitivity analysis of main model outputs to the stochastic inputs was done. Small ruminants are held across the country mainly managed under two major production systems: the crop-livestock mixed system and the pastoral system. The small ruminant population has grown in the past 10 years with an average annual growth rate of 4.6% for sheep and 6.7% for goats. The national average small ruminant population for 2021 was projected at 96.4 (range 95.3–97.7) million heads and the mean stock biomass was about 2,129 (range 1,680–2,686) million kilograms. The monetary value of the small ruminant population was estimated at USD 5,953 (range 4,369–7,765) million. The annual monetary value of small ruminant production outputs was estimated at USD 1,969 (range 1,245–2,857) million. Although the small ruminant population is large and rapidly growing, contributing about 2% of national annual GDP, the sub-sector is characterized by low productivity, low offtake rates, and a limited range of production outputs with no signs of intensification. Efforts should be made to reduce small ruminant mortality, improve fertility, and better utilize products such as milk to improve the livelihoods of rural households and to benefit the national economy. The approaches developed in this study can be replicated in other systems and countries to reveal trends in the size and value of livestock systems, providing a better understanding of its economic importance and performance.

Yin Ke Li

Papers

Changes in soil organic carbon, nutrients and aggregation after conversion of native desert soil into irrigated arable land.

Soil organic carbon and nitrogen fractions and water-stable aggregation as affected by cropping and grassland reclamation in an arid sub-alpine soil

Population, biomass, and economic value of small ruminants in Ethiopia