Localized application of phosphorus and ammonium improves growth of maize seedlings by stimulating root proliferation and rhizosphere acidification

Root growth of many plant species can be stimulated by localized supply of nutrients such as nitrate, ammonium and phosphorus (P). The ammonium supply can cause rhizosphere acidification by inducing proton release from roots, but little information is available about ammonium and P interaction in improving nutrient use and plant growth through modifying rhizosphere processes in the field. In the present study, maize was grown on a calcareous soil (pH 8.1) in an intensive farming system in North China. To test the hypothesis that localized application of P and ammonium can improve nutrient uptake and plant growth through ammonium-induced rhizosphere acidification, the 2-year field experiment was conducted with the following treatments (N supply as ammonium): localized P application with N broadcast (NBPL), localized application of both P and N (NLPL), or both N and P broadcast (NBPB). Localized supply of P plus ammonium significantly increased chlorophyll content and leaf area, leading to increased dry weight of shoots at early growth stages. The leaf expansion rate was 20-50% higher, the total root length 23-30% greater, and the plant growth rate 18-77% greater in NLPL than NBPB or NBPL treatments at early growth stages. The root-length density at the depth of 0-15 cm was greater in NLPL than NBPL or NBPB, with a greater proportion of fine roots (diameter <0.2 mm) in NLPL. Localized supply of P plus ammonium markedly modified rhizosphere processes by stimulating root proliferation (especially of fine roots) and by ammonium-induced rhizosphere acidification, thus significantly increasing acquisition of N and P by plants. Localized application of P combined with addition of ammonium significantly decreased rhizosphere pH (by 3 pH units) in the fertilized zone compared with the bulk soil. The results indicate that localized application of P combined with ammonium can significantly improve maize growth and nutrient use at the early stages by stimulating root proliferation and rhizosphere acidification, suggesting that modifying rhizosphere processes in the field may be an effective management strategy for improving nutrient use efficiency and plant growth. (C) 2010 Elsevier B.V. All rights reserved.