Effects of soil conservation measures on erosion rates and crop productivity on subtropical Ultisols in Yunnan Province, China

Abstract

Soil erosion in Yunnan Province, southwest China, is recognised as a major environmental problem. With some 95% of the Province classed as mountainous and the valleys and plains already fully utilised for agriculture, industry and urban growth, pressure on remaining land resources is high. Cultivation has steadily encroached onto increasingly steep and marginal land, which further accelerates erosion rates. Little published work is available of soil erosion rates or potential soil conservation measures in Yunnan. A collaborative research programme was initiated between Yunnan Agricultural University (YAU) and The University of Wolverhampton to study several potential soil conservation measures. Thirty runoff plots on the experimental farm of YAU were used to investigate the effectiveness of five treatments: conventional tillage (control), no-tillage, straw mulch, polythene mulch and intercropping. Plots were cropped with maize (Zea mays) sown either along the contour or downslope, on 3°, 10° and 27° slopes. Data on erosion rates and crop productivity were collected during four growing seasons (May–October), 1993–1996 inclusive. Seasonal rainfall totals were 576.3, 768.3, 876.3 and 619.7 mm in 1993, 1994, 1995 and 1996, respectively. Respective seasonal mean erosion rates equalled 0.73, 4.00, 7.56 and 0.05 t ha⁻¹. Straw mulch was very effective in decreasing erosion rates. In 1993, 1994, 1995 and 1996, soil loss was 18, 66, 86 and 78% less than the conventionally tilled plots, respectively. Straw mulch maintained topsoil structure and encouraged infiltration, thus decreasing runoff and erosion rates. Conversely, erosion rates under conventional tillage were high. Erosion rates from the polythene mulch plots were similar to conventional tillage, as infiltration was effectively decreased, thereby concentrating runoff and channelling it towards exposed, inter-mulch areas. However, maize development and grain yields were consistently higher under the polythene mulch than the other treatments. Mean contour cultivation erosion rates were 31% less than downslope planting rates. The collaborative research on soil conservation is continuing, with a catchment-based research programme underway in Wang Jia catchment in Kedu Township, in northeast Yunnan.

Introduction

In southern China, water erosion has increased in recent years, although confirmation is difficult due to limited data (Dregne, 1992). Soils in Yunnan Province have been severely damaged and have declined in fertility, due to poor land management, extensive cultivation and lack of conservation (United Nations Environment Programme, 1990). Less than 5% of the land is level (Thomas, 1992) and population pressure on land resources for agriculture, urban growth and industry is continually increasing. The Yunnan plateau has been populated since c.4000 BP (Fullen et al., 1999) and the provincial population in 1996 was ∼40.0 million. The combination of increasing population and food demands has led to the encroachment of cultivation onto increasingly steep slopes, leading to accelerated erosion rates. Forest cover in Yunnan declined from about 60% in the 1960s to 24% in 1990 (United Nations Environment Programme, 1990). Crops such as maize, tobacco and tea are widely grown, often on steep slopes, where runoff is high (Thomas, 1993). Cultivation does not always incorporate soil conservation measures or appropriate agricultural management strategies and consequently, soil erosion is accelerated. To curb soil erosion, legislation states that arable land on slopes >25°, common in Yunnan, should be progressively returned to forestry or grassland (Ni, 1993, Hill, 1995). However, erosion continues to pose problems on remaining arable areas. Therefore, a combination of anthropogenic factors superimposed upon unfavourable natural conditions has accelerated erosion rates. Based on limnological evidence from selected catchments, Whitmore et al. (1994) estimated that erosion in Yunnan has increased 15-fold over recent centuries due to human activities. It is clear that low-cost solutions to erosion problems are needed to maintain soil resources, ensure long term efficient crop production and promote good land husbandary. Thus, the effectiveness of several agronomic and soil management conservation measures need to be assessed.

As part of an ongoing collaborative research programme between The University of Wolverhampton, UK and Yunnan Agricultural University (YAU), China, investigations into soil erosion and conservation in Yunnan Province were initiated in 1993. A number of potential low-cost conservation measures were tested on various slope conditions, primarily to assess their effect on runoff and erosion rates. Treatments were chosen to simulate several typical situations in central Yunnan and to test less well known or unconventional agricultural practices. Treatments included mulching with either straw or polythene, intercropping, no-tillage and contour cultivation.