Redistribution of phosphorus by cattle on a traditional mountain pasture in the Alps

Abstract

Over a period of centuries, grazing cattle on a mountain pasture can produce strong heterogeneities in the distribution of plant species and soil nutrients. To understand how these patterns arise, we observed the activities of a herd of Scottish highland cattle on a subalpine pasture (alp) in southern Switzerland during two grazing seasons, and modelled the phosphorus balance resulting from P removal by grazing and P return in dung. Cattle mainly grazed on the lower parts of the pasture, and their dung was concentrated in a few small patches in these lower areas. A GIS model indicated that about half the site had a P balance close to zero (−0.5 to +0.5 kg ha −1 year −1), while one quarter gained >0.5 kg P ha −1 year −1, and a similar area lost >0.5 kg P ha −1 year −1. However, the spatial distribution of gains and losses was not symmetrical because a few places – mainly where animals gathered to rest and ruminate – gained >50 kg P ha −1 year −1, but none lost P at such a high rate. P transfers were greater in 1998 when cattle could move freely over the whole area than in 1996 when the area was subdivided into two paddocks. Data on the soil P capital suggest that spatial patterns of nutrient transfer on such an extensively used alp are relatively stable, with the consequence that small patches have accumulated high amounts of P, while much larger areas have become depleted. Extrapolating from the model, we show that the highest soil P contents recorded in our study area could have accumulated in just 200 years of grazing, assuming the spatial pattern of fluxes remained constant. We conclude that, by concentrating nutrients in small areas, cattle promote nutrient loss from mountain pastures and thus cause a gradual decline in their productivity. However, simple management practices such as the use of fences to restrict animal movement can help retain the soil nutrient capital of these grazing systems.