ASSESSMENT OF NET NITROGEN MINERALISATION FROM REPEATED APPLICATIONS OF CATTLE-SLURRY AND MINERAL FERTILISER IN A MEDITERRANEAN FORAGE SYSTEM

The aim of our study was to evaluate the impact of repeated applications of cattle-slurry and mineral fertiliser on net N mineralisation rates in a Mediterranean forage system, cropped with maize silage (spring-summer) followed by Italian ryegrass (autumn-winter) for silage. The field study was performed between May-1997 and May-2000 in a commercial dairy-cattle farm located in Northwest Portugal. Five treatments were employed: Control with no N fertiliser (T0); one application of cattle-slurry at maize sowing with 187 (T1) or 340 (T2) kg total N ha-1 year-1; two applications, 258 kg total N ha-1 at maize and 150 kg total N ha-1 at ryegrass sowing plus application of 190 kg total N ha-1 to maize and 50 kg total N ha-1 to ryegrass as mineral fertiliser, totalizing 648 kg total N ha-1 year-1 (T3), and; application of 240 kg total N ha-1 year-1 as mineral fertiliser to both crops like in T3 (T4). Net N mineralisation was measured in soil layer 0-30 cm in each treatment. Significant amounts of N were released (160-290 kg N ha-1 year-1) by mineralisation due cattle-slurry application to soil, with apparent net N mineralisation varying between 45 and 95% of the total N applied. The application of mineral fertiliser led to N immobilisation (-23%) of the total N applied. Data obtained in this study may be used in the management of the N fertilisation to improve manure-N efficiency.