Organic matter in farming systems in Southern Spain by LOI and Walkley-Black methods.

Sustainable agriculture is based on the responsible use of soil resources. Soil organic matter (SOM) is one of the most important properties and would be taken in consideration in any modelization associated to the mitigation of climate change. The estimation of SOM has been widely obtained based on two main methodologies: ignition and oxidation. The method to measure soil organic matter by loss on ignition (LOI) is considered an easy and fast method. However, some interactions depending on the temperature used and the presence of carbonates, can produce overestimations. On the contrary, the Walkley-Black method (WB) is a relatively accurate method based on the oxidation of organic matter but recalcitrant carbon substances can resist this oxidative attack.

Our preliminary study aims to evaluate the relationships between these two methods in calcareous agricultural soil samples taken in the province of Alicante, in the South East of Spain. Land use were divided in three main agricultural uses: horticultural crops, fruit crops and pasture. For this purpose, 41 sites were sampling, 16 samples belong to horticultural systems, 8 to fruit crops and 17 in pasture. The samples were collected at a depth of (0-20cm).

The results of the organic matter content (mean value and standard deviation) expressed in g/kg for LOI (4h. at 380^oC) and WB (traditional method) were the following for each land use: horticultural LOI=114±24 and WB=31±7; fruit crops LOI=97±4 and WB=63±51; and pasture LOI=66±19 and WB=32±17. After that, a simple linear regression was used to compare LOI and WB. The results showed the following: R²=0.31 and p < 0.01; R²=0.74 and p < 0.05; and R²=0.41 and p < 0.001; for horticultural, fruits crops and pasture land use respectively. The relation between both methods was higher under fruit crops. The mean value of carbonates for each land use group were: 112±6 in horticultural soils; 113±6 in fruits cropping system; and 11±4 in pasture. A simple linear regression was used again to compare LOI-Carbonates and WB-Carbonates in horticultural systems, fruit crops and pasture land use. In this case, the Pearson correlations were R²=0.62, p < 0.01 and R²=0.16, p < 0.001; R²=0.08, p < 0.01 and R²=0.6, p < 0.001; R²=0.006, p < 0.001 and R²=0.08, p < 0.01; respectively. No linear dependence between two variables analysed (LOI-Carbonates and WB-Carbonates) was found in any farming system.

The relation between soil organic matter content determined by using LOI and WB, revealed that a good relation was found in the pasture land use, which reflects that in uncultivated soils, organic matter would tend to the stabilization. On the other hand, the relation between soil organic matter content and carbonates, indicates that there is no relationship between them, excepting for the relationship between LOI and carbonates in horticultural soils, which may indicate that carbonates are easily degradable in cultivated soils (under intensive agriculture) and their presence can overestimate or has some influence on the soil organic matter content obtained by using the LOI method. However, more research is need to obtain satisfactory results.