Abstract
Abstract. Much of the native forest in the highlands of western Kenya has been converted to agricultural land in order to feed the growing population, and more land is being cleared. In tropical Africa, this land use change results in progressive soil degradation, as the period of cultivation increases. Both rates and variation in infiltration, soil carbon concentration and other soil parameters are influenced by management within agricultural systems, but they have rarely been well documented in East Africa. We constructed a chronosequence for an area of western Kenya, using two native forest sites and six fields that had been converted to agriculture for up to 119 yr. We assessed changes in infiltrability (the steady-state infiltration rate), bulk density, proportion of macro- and microaggregates in soil, soil C and N concentrations, as well as the isotopic signature of soil C (δ13C), along the 119-yr chronosequence of conversion from natural forest to agriculture. Infiltration, soil C and N decreased within 40 yr after conversion, while bulk density increased. Median infiltration rates fell to about 15% of the initial values in the forest, and C and N concentrations dropped to around 60%, whilst the bulk density increased by 50%. Despite high spatial variability, these parameters have correlated well with time since conversion and with each other.
Highlights
IntroductionSoil infiltrability (defined as the infiltration rate when water at atmospheric pressure is made freely available at the soil surface) (Hillel, 1971) and soil carbon (C), the major part of soil organic matter (SOM), are interrelated parameters that largely determine agricultural productivity
Soil infiltrability (Hillel, 1971) and soil carbon (C), the major part of soil organic matter (SOM), are interrelated parameters that largely determine agricultural productivity
Infiltrability is a reflection of soil structure and texture (Cresswell et al, 1992), soil biological activity (Mando, 1997; Leonard et al, 2004), soil aggregation (LeBissonnais and Arrouays, 1997) and SOM content (Franzluebbers, 2002)
Summary
Soil infiltrability (defined as the infiltration rate when water at atmospheric pressure is made freely available at the soil surface) (Hillel, 1971) and soil carbon (C), the major part of soil organic matter (SOM), are interrelated parameters that largely determine agricultural productivity. A high infiltrability enables water to enter the soil to become available for plant uptake and allows ground water recharge. Biological activity enhances soil aggregation (Jastrow et al, 1998), aeration, water holding capacity and infiltrability. Soil bulk density is often correlated with both soil C and with infiltrability (Mbagwu, 1997; Mariscal et al, 2007; Arvidsson, 1998) These three parameters (infiltrability, soil C and soil bulk density) are suitable for studying changes in soil fertility and production capacity (Doran and Parkin, 1994)
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