Long term sugarcane straw removal affects soil phosphorus dynamics

Brazil is the world's largest sugarcane producer. This crop leaves huge amounts of straw behind which have to be managed with innovative approaches. In this study, a field experiment was carried out over six successive years of sugarcane harvests to evaluate the effect of straw maintenance on the field on changes in soil phosphorus (P) fractions. Four straw maintenance rates were tested: (i) maintaining 15 Mg ha−1 (SM15); (ii) maintaining 10 Mg ha−1 (SM10); (iii) maintaining 5 Mg ha−1 (SM5), and (iv) maintaining 0 Mg ha−1 (SM0). Our results showed that low maintenance of straw (SM5) resulted in the highest amount of P extracted by anion exchange resin and inorganic P extracted by 0.5 mol L−1 NaHCO3 in 0−5 and 5−10 cm soil depths. While total removal of residues resulted in soil P deficiency after six crop cycles, keeping straw on the soil surface at the rate of 5 Mg ha−1 supports plant growth without the need for P fertilizer application. We detected inorganic P extracted by 0.1 mol L−1 NaOH as the sink of P derived from straw since it was the only P fraction which increased with increasing straw on the soil surface. Non-labile P fractions were not influenced by straw maintenance rates in all depths. This is a promising result since it shows that P derived from straw does not accumulate in non-labile P fractions which are not plant available. Inorganic P was not influenced by straw removal rates and total P was only changed in 0−5 cm soil layer, while organic P was affected in all depths. We observed that organic P is available to sugarcane plants under SM0, SM5, and SM10, while under SM15, organic P does not play a role in sugarcane nutrition.