Aggregate distribution pattern of available phosphorus, acid and alkaline phosphatase activity in different slope positions

There is a close relationship between microbial activities and soil physical environment. The specific location of each enzyme within the soil matrix has been attributed to the location of their substrates, enzyme characteristics or the organisms responsible for their production. Topography (slope position), one of the important soil forming factors, plays a significant role in development, evolution and variability of soils. Landscape-scale variations in soil enzyme activities are controlled by pedological and hydrological processes. The effect of topography (slope position) on aggregate size distribution and the enzyme activities has been evaluated in previous studies. Slope position may influence the aggregate distribution of acid and alkaline phosphomonoesterase differently. Therefore, the objective of this research is to evaluate the aggregate distribution of acid phosphatase and alkaline phosphatase activities in different slope positions.
Soil samples were collected from three slope positions of a grassland ecosystem. At each landscape position, three equally spaced points were selected, at a distance of 10 m. In each point, ten soil cores of 0–10 cm depth were taken within a 2 m radius and then composited. After measuring general properties in soil, aggregates were separated by wet sieving method. The aggregates remained on each sieve were collected and weighted. Organic C, available phosphorous, acid phosphatase and alkaline phosphatase were measured in the six size ranges (4–2, 2–1, 1–0.5, 0.5–0.25 and 0.25–0.05,
Results showed that aggregate size distribution was influenced by slope position. Microaggregates were lowest and greatest in summit and backslope positions, respectively. Macroaggregate contents were related by organic C along slope position. Macroaggreagtes decreased in backslope positon by 85%. The results also showed that organic C and available P were greater in macroaggregates than microaggregates by 38 and 12% respectively. Both acid phosphatase and alkaline phosphatase activities were greater in macroaggregates than microaggaregates and their activities decreased with decreasing aggregate size by 42%. The effect of slope positon on the distribution pattern of phosphomonoesterase was different. Acid phosphatase and alkaline phosphatase activities were lowest in toeslope and backslope, respectively. Although, acid phosphatase to organic C and available P was higher in all aggregates of backslope position, the distribution pattern of alkaline phosphatase to organic C and available P in different slope positions was dependent to aggregate size. Alkaline phosphatase was only related to available P (r=0.72 P=0.001).
In overall, acid and alkaline phosphatase activities had similar distribution in soil aggregates but both enzymes were distributed differently in different slope positions. Since macroaggregates contain higher organic C, available P and phosphomonoesterase activities, soil structure protection of sensitive slope positions to erosion can influence on P cycling in soil.