Nitrogen mineralization in soils amended with composted olive mill sludge

The disposal of olive mill wastewater (OMW) is a critical pollution problem, especially in Mediterranean countries. OMW is produced at a rate of ca. 2 1 kg-1 of olives. OMW has a biochemical oxygen demand (BOD) of 35–48 g kg-1. The evaporation of OMW in ponds produces large amounts of sludge (OMWS), which after being dried can be used as organic fertilizer or amendment, either directly or co-composted with agricultural by-products. The present paper deals with the use of a compost of dried sludge of OMW and other agricultural by-products – olive mill wastewater sludge compost (OMWSC) – to amend for three consecutive years two typical soils of southern Spain: a Typic Xeropsamment, S1 (CaCO3 86 g kg-1; OM 1.4 g kg-1; sandy soil) and a Typic Xerorthent, S2 (CaCO3 270 g kg-1; OM 5.3 g kg-1; sandy–clay–loam soil). Both soils are poor in organic matter, in total Kjeldahl-N (TKN 347 and 536 mg kg-1) and in available-P (Olsen P 3.0 and 2.5 mg kg-1). Mineralization of the organic-N of the OMWSC (OM 106–338 g kg-1; TKN 7–15 g kg-1; C/N 12.3–18.9) and of soils treated with OMWSC was studied under laboratory conditions by an aerobic, non-leaching incubation procedure. In both cases, the N-mineralization process was well described by first-order-reaction kinetics, and the potentially mineralizable-N (N0) and the N-mineralization rate constant (k) estimated from a single first-order equation. Values for N0 (equivalent to 16% of the N of the compost) and k (0.041 week-1) indicated that the OMWSC was a mature compost composed mainly of well-humified organic matter, very resistant to mineralization. Soils treated with OMWSC showed increases of the OM, TKN and N0 contents and of the potential rate of mineralization (N0 x k), with respect to untreated or mineral fertilized soils. The increases of TKN in soils treated with OMWSC were even higher than expected by the N added together with compost, and attributed to non-symbiotic N fixation. Values of OM, TKN, N0 and N0 x k, were higher in S2 than in S1, which can be explained based on the soil properties related to fertility, especially soil texture.