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COPYRIGHT 2002 CSIRO Publishing
Introduction
Bulk density is a key parameter used in many soil hydrologic models for predicting a wide range of soil processes. The influence of many soil management practices on the soil properties that affect water movement and retention in soils requires information on the soil bulk density. However, bulk density is not always determined in routine soil surveys, as field procedure deployed for its determination is tedious, labour-intensive, time-consuming, and expensive. In addition, due to the spatial variability of soils, a large number of soil core samples is required to adequately represent a large area or a watershed. In fact, lack of this information at a regional scale has impeded both the application and improvement of the soil hydrologic models.
Experience has shown that organic carbon (OC) and texture predominately determine soil bulk density. Organic carbon and texture information is often available in soil survey campaigns. Therefore many attempts have been made to estimate soil bulk densities through some pedo-transfer functions (PTFs) based on soil OC and texture data (Curtis and Post 1964; Adams 1973; Alexander 1980; Federer 1983; Rawls 1983; Huntington et al. 1989; Manrique and Jones 1991; Bernoux et al. 1998; Tomasella and Hodnett 1998). Most of these PTFs, except the ones developed by Rawls (1983), Tomasella and Hodnett (1998), and Bernoux et al. (1998), are a function only of organic matter (OM)/OC content. Although studies conducted by Saini (1966) and Jeffrey (1970) have shown that OM has a dominating effect on soil bulk density and that it alone can be used as a good predictor of soil bulk density, it has been observed (e.g. Alexander 1980; Huntington et al. 1989; Manrique and Jones 1991) that soil texture plays a major role in controlling bulk density where OM is a minor component. Further, no attempts have been made to date to test and compare the applicability of these PTFs on an independent soil data set. In addition, no studies on the effect of varying levels of soil particle size distribution and/or OC contents (i.e. input parameters) on the errors associated with these estimation methods (i.e. sensitivity analysis) have been attempted so far for testing their applicability to a wide range of soil types.
Thus, the objectives of this study were to:
(i) develop a soil bulk density estimating PTF, based on both soil texture and OC content data; and
(ii) test/compare the applicability of both proposed and existing PTFs on an independent soil data set.
Materials and methods
The objectives set forth for the study necessitated the collection of a large number of soil samples from 4 ecologically diverse (i.e. agricultural, pine forest, oak forest, and barren) micro-watersheds situated between 29[degrees]34'05" and 29[degrees]38'20" N latitude and 79[degrees]32'15" and 79[degrees]36'10" E longitude in Almora district of Uttaranchal State in India. The total area, average slope, and absolute elevation of these test-watersheds (Fig. 1) ranged between 17 and 66 ha, 24[degrees] and 28[degrees], and 1220 and 2190 m above m.s.1., respectively.
[FIGURE 1 OMITTED]
Soil sampling strategy and analysis
Soil samples (224) were extracted from the top and the subsoil horizons of 58 sampling sites located within the test-watersheds. Care was taken to select sampling sites with varying land-use along the contours of each test-watershed. Overlying of these sampling sites on the top and the subsoil depth maps of test-watersheds (Rawat et al. 1999) led to the following depth-wise soil sampling strategy. As the test watershed soils were neither gravelly nor stony, standard volume-coring method was used for their bulk density estimations. From the sampling sites with moderately shallow (28-110 cm) and shallow (8-16 cm) soil depths, soil core samples were extracted in duplicate from the center of each of the top and the subsoil horizons using a core sampler, with core dimensions of 8 cm height by 7 cm diameter, and 6 cm height by 4.6 cm diameter, respectively. From the sampling sites with very shallow soil depths (5-6 cm), single-layer samples were extracted in duplicate from the middle of the top and subsoil horizons using core dimensions of 4 cm height by 6 cm diameter. After the measurement of bulk density of all these (non-destructive) soil core samples, soil texture (Bouyoucos 1962) and OC contents (Walkley and Black 1934) of the above (destructive) soil samples were determined as per the standard procedures.
A perusal of literature revealed that out of all the existing test-PTFs (see Table 3), the PTFs developed by Curtis and Post (1964), Adams (1973), Federer (1983), Rawls (1983), and one of the PTFs developed by Huntington et al. (1989) were dependent...
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