Impact of an integrated no-till crop–livestock system on phosphorus distribution, availability and stock

Biophysical and socio-economic improvements of farming systems have been obtained with integrated crop–livestock systems (ICLS). Grazing in ICLS under no-tillage conditions may alter soil phosphorus (P) dynamics, because of changes in P budgets and distribution of P forms. The need to understand impacts on P dynamics is important because they can modify soil quality and, consequently, have implications for agriculture sustainability. The objective of this work was to compare the geochemical and biological forms of P (P-geo and P-bio) under long-term grazing, as well as to assess the mobility and lability of these nutrient forms in the soil profile. The experiment was established in 2001 in an Oxisol and consisted of soybean (Glycine max L.) rotated to a winter cover crop of black oat (Avena strigosa Schreb.) and Italian ryegrass (Lolium multiflorum Lam.) under no-tillage management. Treatments were two grazing heights (target of 20 and 40cm) with young cattle and a non-grazed (NG) treatment. The experimental design was set up in completely randomized blocks with three replicates. Soil was sampled at the beginning of the experiment (May 2001) and again after six years (May of 2007) to study P fractionation at depths of 0 to 2.5, 2.5 to 5.0, 5.0 to 7.5, 7.5 to 10, 10 to 15 and 15 to 20cm. After six years of ICLS, total P increased in the top 20cm soil layer in both the grazed and non-grazed treatments. The P-geo and P-bio concentrations also increased in all treatments in the 0 to 2.5-cm layer. Despite the greater P accumulation in the non-grazed no-till treatment, grazing maintained the P lability (labile+moderately labile) at the same level as in the long-term no-till treatment. The ICLS also promoted increases in labile P stocks, both in organic and inorganic forms. In spite of the higher P-geo stocks in the non-grazed treatment, P cycling was enhanced by grazing, such that soybean yields were similar in all treatments. Furthermore, the grazing treatment led to higher P budgets (surpluses) and more efficient P use, as represented by meat production in addition to the soybean production.