Abstract
Abstract. Composite agricultural systems with permanent maize cultivation in the uplands and irrigated rice in the valleys are very common in mountainous southeast Asia. The soil loss and fertility decline of the upland fields is well documented, but little is known about reallocation of these sediments within the landscape. In this study, a turbidity-based linear mixed model was used to quantify sediment inputs, from surface reservoir irrigation water and from direct overland flow, into a paddy area of 13 ha. Simultaneously, the sediment load exported from the rice fields was determined. Mid-infrared spectroscopy was applied to analyze sediment particle size. Our results showed that per year, 64 Mg ha−1 of sediments were imported into paddy fields, of which around 75 % were delivered by irrigation water and the remainder by direct overland flow during rainfall events. Overland flow contributed one-third of the received sandy fraction, while irrigated sediments were predominantly silty. Overall, rice fields were a net sink for sediments, trapping 28 Mg ha−1 a−1 or almost half of total sediment inputs. As paddy outflow consisted almost exclusively of silt- and clay-sized material, 24 Mg ha−1 a−1 of the trapped amount of sediment was estimated to be sandy. Under continued intensive upland maize cultivation, such a sustained input of coarse material could jeopardize paddy soil fertility, puddling capacity and ultimately food security of the inhabitants of these mountainous areas. Preventing direct overland flow from entering the paddy fields, however, could reduce sand inputs by up to 34 %.
Highlights
Paddy cultivation is one of the most long-term sustainable cropping systems, as irrigated rice is the only major crop cultivated in monoculture for centuries without severe soil degradation (Bray, 1986; von Uexkuell and Beaton, 1992)
As nutrient content of sediments is closely related to sediment particle size and puddling is favored by high clay contents (De Datta, 1981), the potential for long-term sustainable rice production is related to the soil texture in paddy fields
The proportion that remained behind was mostly sandy, altering the soil texture in the experimental paddy plots. While these results indicate that paddy fields act as a net sediment trap, their function might differ when upscaled to a larger area as sediment deposition changes over cascade length (Schmitter et al, 2010)
Summary
Paddy cultivation is one of the most long-term sustainable cropping systems, as irrigated rice is the only major crop cultivated in monoculture for centuries without severe soil degradation (Bray, 1986; von Uexkuell and Beaton, 1992) Two mechanisms facilitate this continuing productivity: first, flooding applies suspended particles and soluble nutrients to the fields, and those contribute to the indigenous nutrient supply (Dobermann et al, 2003; Schmitter et al, 2011). Irrigated paddy fields are not isolated elements in a landscape, as they are connected to surrounding upland areas They receive sediments from those upland areas, both directly through overland flow, and indirectly from irrigation water released through surface reservoirs (Schmitter et al, 2012).
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