Soil chemical and microbiological properties affected by land use, management, and time since deforestations and crop establishment. The case of wheat (not irrigated) and cotton (irrigated) fields in filyria, kilkis, greece

Resumen

Deforestation is a common practice in many countries worldwide, in order to gain land for agricultural purposes. The conversion from natural vegetation to cropland leads to a depletion of the soil organic carbon (SOC) stock due to reduced input of biomass and enhanced decomposition after physical disturbance. In a part of Northern Greece, Filyria, three deforestations took place in 1933, 1971 and 1980. The natural vegetation consists mainly of Quercus pubescens stands. The soils of the study area have developed from limestone, and are classified as Xeralfs according to Soil Taxonomy. The plots of the study are cultivated with cotton and wheat. Wheat fields are not irrigated and their management is considered as reduced tillage, whereas cotton plots are irrigated by sprinklers, and considered as conventional tillage. The big amount of time transcurred since the first deforestation, the availability of three different deforestation dates and the remaining undisturbed forest, offer an interesting opportunity of studying these changes, by measuring and assessing microbial biomass carbon (MBC), SOC, the MBC/SOC ratio, soil respiration (SR), the metabolic quotient (qCO2), and the relationship between SOC, N, C/N and the soil fractions. Land use (forestry vs agriculture), type of crop management (wheat vs cotton) and the amount of time since deforestation (26, 35 and 73 years since deforestations) are the factors discussed. Three sampling categories referring to deforestation year in the cultivated fields and one category referring to the natural remaining forest were considered. For each deforestation year category, two crop subcategories were considered (wheat and cotton) microbiological and chemical properties at whole soil samples, and chemical properties at soil particle-size fractions were studied. The results showed significant differences for the values of SOC, MBC, MBC/SOC, SR, qCO2, between forest and agricultural plots. Higher values for forests were detected for SOC, MBC and SR. Tillage promotes soil organic matter (SOM) decomposition, decreasing the sources of energy and the available substrates for the microorganisms, leading to decreases in MBC, and SR. Factors that were significant for these three properties were changes in land use and the amount of years since deforestation. For SOC and MBC, management as a factor was also significant. Forest values for the two microbiological ratios (MBC/SOC and qCO2) had significantly lower values. These lower values for the forest soils of the ratios are indicators of less stressed conditions of the microbial community, due to better preserve of SOC and higher accumulation of microbial biomass. For these ratios, land use was always a significant factor. For the MBC/SOC ratio the amount of years since deforestation was also significant, whereas for qCO2 the management factor was significant. The newest deforestation (1980) had the biggest values for every property, except for the ratios. The amount of time since deforestation was adversely associated with SOC contents. Continuous soil tillage induces C loss and releases large amounts of CO2 to the atmosphere. After many years systems reach or are near an equilibrium, leading to lesser values of qCO2. Wheat plots had always significantly higher values than cotton plots, when referring to SOC and MBC, whereas for qCO2 the reverse trend was observed. The reduced tillage practices of wheat crops have improved conditions of the soil to protect SOC, compared to the conventional tillage practices of the cotton plots. For the other two properties management was not a significant factor. Nitrogen and the C to N ratio in the whole soil, and particulate organic matter (POM) followed the same trends as SOC, being always significantly higher in forest (referring to land use), and in the most recent deforestation (1980). The ratio in forests refers to more fresh material resulting in a higher value. In the forest soils most of SOC and N were present in the fine sand fraction, which is expected. The reverse trend was observed for the agricultural soils, where clay had the (significantly) greater values, due to protection of SOM. The C/N ratio holds its (significantly) greatest values for forest in the fine sand fraction, whereas for the agricultural plots in the silt fraction. Land use was a significant factor for all the properties. Forest soils had the greater values for all the properties in all the fractions compared to the agricultural plots, and except the N contents in clay, the differences were significant. Cultivation results in the breaking up of the aggregates, leading to protected C losses from silt and clay. Soil organic matter losses lead to N losses lead to N losses. The amount of years since deforestation affected significantly C and the ratio, but not N. Nitrogen was mostly dependent on management practices. The most recent deforestation (1980) had the greater values for all the properties in all the fractions. For all the deforestation categories, clay had significantly higher values referring to C and N, whereas for the ratio the significantly higher values were found in the silt fraction. The stabilization of organic matter in the clay particles results in a steady content throughout the years, so we observed similar values for the C/N ratio in this fraction. Management affected the properties in all the fractions, not always significantly, though overall it was a significant factor. The biggest values of SOC and N were always found in clay, but for the C/N ratio in silt. The SOC contents in the clay fraction of the fields had no significant differences regardless management type, due to the stable behaviour of this fraction. Clay in the wheat crops showed always significantly higher values for N, but not in the other fractions. Enhanced N fertilization of the wheat fields resulted in accumulation of N on the clay particles of those fields (also resulting in lower C/N values). Deforestation and further establishment of agricultural use leaded to decreases in SOC and levels of microbiological properties, suggesting a deterioration of soil quality. Land use changes had significant effects in SOC and N contained in the soil fractions. Irrespective of the management that follows, the most significant factor was the land use change for the majority of the studied properties in the soil particle fractions.