Distribution of carbon, nitrogen and phosphorus in water-stable aggregates of an organic waste amended Ultisol in southern Nigeria

This study was undertaken to evaluate the effects of different organic and inorganic amendments on nutrient (C, N and P) distribution in water-stable aggregates (WSA) of a degraded tropical Ultisol at Nsukka in southern Nigeria. The treatments were control (C), inorganic fertilizer (F) (N–P–K, 12–12–17), rice-mill wastes (RW), RW + F, poultry manure (PM) and RW + PM. These were arranged in a randomized complete block design (RCBD) with four replications. At 0, 3, 6 and 12 months after application of the amendments, topsoil (0–20 cm) samples were collected from all the treatments, dried and separated into 4.75–2.0, 2.0–1.0, 1.0–0.5, 0.50–0.25 and <0.25 mm aggregates classes by wet sieving. The mesoaggregates fraction (1.0–0.50 mm) made up the highest percentage of the whole soil irrespective of the treatments. From the sixth month, the treatments generally reduced the proportion of these mesoaggregates as well as those of the microaggregates (<0.25 mm diameter) while increasing those of the other sizes. The largest (>2.0 mm) and the smallest (<0.25 mm) aggregates were preferentially enriched in C and N relative to the 2.0–0.25 mm aggregates in all the treatments, whereas P distribution within the aggregates was random. There was a low, but significant correlation between organic carbon and mean-weight diameter (MWD) of the aggregates. The correlation between C and N within the aggregates was also positive (r=0.917) and highly significant (P<0.01). The implication of these results for management of these soils is discussed.