Soil texture and tree coverage influence on organic matter

Soil organic matter (OM) is an important factor for maintaining the productivity and the stability of the so-called Caldenal woodland ecosystem of central Argentina. Little is known about the influence of tree coverage and soil texture on OM accumulation in soils of the study region. The relationships between OM content, silt + clay content, and the percent of tree coverage were studied in a 25 x 25 m grid in 2 sites. Results showed that OM was lower (P < 0.05) at Site I (2.86%) than at Site II (6.41%). The OM was positively correlated with the percent of tree coverage in Site I (OM = 0.0156 trees cover + 1.97, R2 = 0.29, P < 0.001), but not in Site II. Conversely, a positive correlation with silt + clay was observed in Site II (OM = 0.17 silt + clay -6.79, R2 = 0.41, P < 0.05) but not in Site I. A multiple regression analysis [OM = -16.64 + 0.71 (silt + clay) + 0.04 (tree coverage), n = 78, R2 = 0.55, P < 0.001] tended to confirm these results. Although they differed in their relative weight both silt + clay and tree coverage affect OM accumulation at both sites. The larger influence of trees on OM variability at Site I was attributed to the lower, but highly variable, tree coverage, and the stronger influence of silt + clay on OM variability at Site II can be attributed to the lower variability of tree coverage, and the higher variability of soil texture. According to our results, (a) 41% of OM variability is explained by silt + clay content when tree coverage is dense and uniform, and (b) a 29% of OM variability is explained by tree coverage when such coverage is sparse and heterogeneous. The influence of tree canopy on OM distribution could be explained by its effect on the temperature regime. The influence of soil texture on OM could be attributed to its effect on the water retention capacity of soils. While forest clearing increases the maximum temperature and the temperature amplitude of soils, erosion in cleared areas affects soil texture, and consequently, its water retention capacity. The deterioration of both temperature and soil water regimes deteriorates, in turn, the capacity of the soil to accumulate OM.