Long term changes in soil properties and enzyme activities after almond shell mulching in avocado organic production

Abstract

The recycling of almond shell as mulch would allow avocado orchards to be managed organically, maintaining plant yield and improving soil conditions simultaneously. This study aims to analyze the long term effects of successive applications of almond shell as mulching for organically grown avocado. Effects on soil properties, enzyme activities and soil carbon storage as well as avocado yield and growth were studied in comparison to no-tillage conventionally managed (using mineral fertilizers and herbicides) avocado. In 2002, 2007 and 2012 almond shells were applied in a 7 cm thick layer (26.0 kg dry matter m−2) covering 4 m× 8 m per tree. After a 10-year period a new soil organic horizon was established in the organic treatment, and the upper mineral soil layer (0–25 cm) enhanced its C content from 43 g kg−1 in F to 91 in g kg−1, and proportionally P and N contents. The high lignin content of almond shells, 362 g kg−1, led to low organic matter degradation, 37% for the 10-year period. Nevertheless, this reduced rate of organic matter mineralization was sufficient to supply the needed nutrients and to maintain or even increase fruit yield in the organic treatment. Urease, β-glucosidase, dehydrogenase, protease and phophomonoesterase showed high microbiological activity in the organic layer decreasing in mineral layers. Enzyme activities were significantly correlated to soil OC (organic C) content and site specific soil moisture. Reduced mineralization and high surface microbiological activity make almond shell an ideal mulch for avocado trees combining increased soil organic C content and agricultural benefits.