Investigation by electro-ultrafiltration on N and P distribution in rhizosphere and bulk soil of field-grown corn

The distribution of available levels of N and P in rhizosphere and bulk soils of field-grown corn (Zea mays cv. Forban 300) in response to N, P, K fertiliser supply was investigated by electro-ultrafiltration (EUF). This technique allowed 3 operationally defined nutrient fractions to be extracted: soluble and immediately available (EUF-I), available (EUF-II), and retained reserve (EUF-III). For nitrogen, the NO3– and NH4+ forms were measured in the EUF extracts. The investigation was carried out providing N, N+P, N+K, and N+K+P. The results indicated that only at 40 days after sowing (DAS), rhizosphere soil was significantly less alkaline than bulk soil and characterised by higher organic carbon levels that increased with crop age. The slightly more acidic environment of the rhizosphere at 40 DAS seemed to be related to the lower levels of EUF-N-NH4+ fractions of rhizosphere soil (0.1–3.2 mg NH4+/kg) relative to bulk soil (0.5–4.9 mg NH4+/kg), with more significant differences observed for the soluble NH4+ pool, EUF-I, and when N was combined with P (N+P) or K (N+K). The extensive nitrification of NH4+ and the initially greater availability of NO3– accounted for the larger extraction of NO3– in the EUF-I-NO3– fraction at 40 DAS. The levels of labile extracted NO3– were ~8 times greater than EUF-II and EUF-III fractions, with significantly higher values of 27.4 (rhizosphere soil) and 18.8 mg NO3–/kg (bulk soil) for the N+P treatment. Due to plant uptake, at 40 DAS the EUF-I-NO3– fraction of rhizosphere soil was also the only pool significantly (P < 0.05) lower than bulk soil, with differences that for N+P treatment were in excess of 45%. Later, at 60–120 DAS, during maximum corn nutrient demand, the levels of all of the EUF-N-NO3– fractions became higher in bulk soil. The pool of labile P, EUF-I-P, was higher in the rhizosphere soil and related to pH lowering, which occurred especially at 40 DAS and fell below the adequate level indicated by the ratio EUF-II-P/EUF-I-P. The use of the EUF methodology allowed a rough estimate of the amounts of different available nutrients of the studied soil as well as of their relationships with soil properties. Issues of rhizosphere sampling in situ were considered and discussed.