40 years of succession or mulching of abandoned grassland affect phosphorus fractions in soil

- P speciation in grassland soil changes within decades.
- Over time stable P fractions increase at the expense of labile P fractions.
- Mulching decelerates temporal changes in P speciation in soil.

Grazing and mowing commonly form part of agricultural management. Nowadays, these grassland management measures - complemented by low-cost treatments such as mulching - often aim at keeping the rural landscape open and at avoiding natural succession. The evaluation of these treatments has not yet accounted for the long-term perspective particularly important for soil phosphorus (P).In a four-decade long experiment in Southwestern Germany, the effects of mulching and natural succession on labile, moderately labile, and stable inorganic (Pi) and organic (Po) P fractions in soil of six sites (three in the Black Forest and three at the Swabian Alb) were studied. Our objectives were to test i) the effect of mulching or natural succession on P fractions in soil, and ii) whether the temporal development of P fractions in soil differs among mulching treatments and natural succession up to 40 years after abandonment of former land use. Special emphasis was paid to differences in bedrock combined with water regime.Soils developed on non-calcareous bedrock showed higher labile Po concentrations as compared to those developed on calcareous bedrock caused by reduced decomposition induced by water saturation and low quality of plant material. The normalization of P fractions to the respective site average revealed that mulching twice a year reduced concentrations of labile and moderately labile Pi, stable and total P in soil likely attributable to vertical transport of Pi. Comparing soil P fractions in 2014 to those of archived samples of 1975, succession resulted in a decrease in labile P fractions and increase in stable P fractions over time which could be explained by processes during soil development. Such processes were less pronounced in the mulching treatment indicating that decomposition of mulching material contributes to maintain P nutrition. In conclusion, mulching can be recommended as it contributes to maintaining P fractions bioavailable while at the same time promoting P limiting conditions.