Developing a sustainable agro-system for central Nepal using reduced tillage and straw mulching
Introduction
Accelerated soil erosion has been a chronic problem ever since the dawn of settled agriculture and has adverse economic and environmental impacts (Lal, 1998). For rainfed agro-ecosystems in Nepal, soil erosion is most severe during the pre-monsoon season (Ries, 1995; Schreier and Shah, 1995; Sherr and Yadav, 1996; Tripathi et al., 2000a; Schreier et al., 2001; Atreya, 2002; Gardner and Gerrard, 2003; Atreya et al., 2006). During the pre-monsoon season, rainfall events tend to be of high intensity accompanied by strong winds and occasionally hail. Further, before the onset of pre-monsoon, the agricultural lands are tilled several times, which makes soil dry, bare and devoid of vegetative cover. Thus, soil and nutrient losses from the upland terraces are usually high even before the onset of actual monsoon. Gardner and Gerrard (2003) clearly state that “in the case of middle hills, [soil and nutrient] losses will be seriously underestimated if the small number of high magnitude storms in late April and May after plowing but before development of weed cover, are not measured”. The rate of soil loss gradually diminishes as the soil cover from maize (Zea mays) crop and other vegetation increases following the onset of the monsoon season.
There is a growing literature that indicates conventional tillage to be the dominant practice leading to accelerating soil erosion especially in slopping farms of the Himalaya. In the Indian Himalaya, Sharma et al. (2001) and Jain et al. (2000) found very high rates of overland flow, soil and nutrient losses from bare agricultural fields following traditional practices compared to other land uses. In Nepal, the tillage intensity is a major factor that enhances soil erosion during pre-monsoon in upland agro-ecosystems. Thapa (1996) argues that farmlands have been undergoing unsustainable rates of soil erosion and soil nutrients depletion due to frequent hoeing and plowing. Farmers believe that the exposure of land to the sun, rain and air for a long period helps to improve soil fertility (Thapa and Poudel, 2002). Therefore, they plow lands immediately after the harvest of crops and leave them exposed without any vegetative cover. Atreya et al. (2006) pointed out that Nepalese farmers in central hills till land three times before maize sowing viz. (i) before the onset of pre-monsoon season (nearly 2 months earlier, February/March), (ii) after 20–25 days (March/April) of first tillage, and (iii) at the time of maize sowing, which is after 2 to 3 pre-monsoon rainfall events (April/May). Due to this tillage operation, soils become extremely dry and dusty, without any vegetative cover allowing the high intensity pre-monsoon rain wash away a significant portion of top soil.
As alternatives to the conventional tillage system, various studies (Acharya, 1999; Rajbhandari, 2000; Bajracharya, 2001; Ransom and Rajbhandari, 2001; Gardner and Gerrard, 2003) reported minimum tillage and mulching to be potential practices for mitigating soil erosion during pre-monsoon. Tripathi et al. (2000b) also recommended developing options such as mulching and reduced tillage in order to enhance sustainability of agriculture in the Himalaya. Use of crop residue mulch has profound beneficial effects on soil properties, microclimates, and agronomic productivity. Mulching conserves soil and water, improves soil structure, improves soil organic matter content, adds cations to the soil, regulates soil temperature, and restore the productivity of degraded lands (Srivastava et al., 1993). Mulching dissipates raindrop energy and thereby decreases soil detachment. It also encourages infiltration rates by minimizing surface sealing, and thus reduces runoff and soil loss (Adekalu et al., 2007). Edwards et al. (2000) reported that mulching of barley (Hordeum vulgare) straw at 4 t/ha reduced soil loss by almost 50% compared to control. Tolk et al. (1999) also noted that mulching at 6.7 t/ha increased grain and stover yields of maize by 17% and 19%, respectively, compared to bare treatment. Similarly, Smolikowski et al. (2001) and Barton et al. (2004) also reported reduced runoff and soil loss by mulching, but no effects on crop yields. Reduced tillage also decreases runoff through reducing surface crusting and increasing infiltration (Cassel et al., 1995; Acharya, 1999; FAO, 2000). The practice of crop residue retention and minimum tillage, in association with basal fertilizer application, increases the supply of carbon, nitrogen and enhances the degree of synchronization between crop demand and nitrogen supply. Kushwaha and Singh (2005) pointed that retention of a small fraction of above-ground biomass and its incorporation in the soil through minimum tillage enhances crop productivity and soil fertility in a cereal based tropical dry land agro-ecosystem.
The practice of covering the plowed land by crop residues for soil and nutrient conservation has been documented in hills of Nepal (Paudel and Thapa, 2001; Khanal and Watanabe, 2006). However, only farmers with a relatively large household size and small land holdings used this method as availability of residue was a constraint for others (Paudel and Thapa, 2001). Applications of mulching and reduced tillage for soil and nutrient conservation during the pre-monsoon season have been recommended as potential researchable options in Nepal. However, to date very few field experiments have been conducted on reduced tillage and rice-straw (Oryza sativa) mulching. Further, crop yield in sloping upland has been in decline in most areas, at the same time farmers continue to cultivate despite negative benefit–cost ratios as there are few other alternatives for livelihood. There is an urgent need to increase/stabilize crop yields and reverse this trend. Therefore, this study evaluated the effect of the two conservation practices, namely reduced tillage and mulching on soil and nutrient losses and maize yield in maize – mustard (Brassica juncea) and maize+soybean (Glycine max) – mustard agro-ecosystems.
Section snippets
Runoff plots and experimental design
Twelve runoff plots (two replicates), each of 6 m×3 m with an average slope of 18% and northeast aspect, were established within the Kathmandu University premises (1500 m above sea level) to measure soil and nutrient losses and crop yield during 2001. Another 18 productivity plots (three replications) were also setup in nearby farmers’ field, each of dimensions 3 m×3 m, for crop yield only. This was done basically to increase the replication and thus, to minimize the experimental errors for crop
Precipitation at experimental site
The amount of rainfall received in different months is presented in Fig. 3. Total rainfall received were 1445 and 2143 mm in 2001 and 2002, respectively, indicating a dry year in first and wet year in the latter. During pre-monsoon season (March–May), 15% of annual rainfall occurred in 2001 and 16% of annual rainfall in 2002. Total numbers of rain events that eroded soil from the runoff plots were 30 in 2001 and 24 in 2002. Of these, only seven (in 2001) and 11 (in 2002) events occurred during
Conclusion and recommendations
Mulching and reduced tillage significantly lowered soil and nutrient losses compared to conventional tillage. Soil nitrogen and organic matter losses, as particulate organic matter associated with eroded sediment, was quite high in conventional tillage. Thus, there is an urgent need to modify conventional tillage practices to protect plant available nutrients in agricultural land, especially in hills of central Nepal, which has 42% of the total farmland and 44% of the population of the nation.
Acknowledgments
Authors are thankful to International Maize and Wheat Improvement Centre (CIMMYT), South Asia Regional Office, Kathmandu, Nepal for the financial support. We are also grateful to J.K. Ransom, B.P. Tripathi, and P.B. Shah for their support during the study period.
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