No abstract

Current limitations on the use of conventional N fertilizers on sugarcane (Saccharum spp.) in Brazil require the search for alternative sources with adequate N use efficiency. Field experiments were conducted in Typic Hapludox (TH) and Typic Eutrustox (TE) soils in São Paulo State, Brazil, to evaluate the dynamics of soil mineral N, N uptake, and sugarcane yield in response to N amendments including Ajifer‐8, ammonium chloride, ammonium nitrate, calcium ammonium nitrate (CAN), and urea applied on sugarcane ratoon at 100 kg N ha−1. The experiments were arranged in randomized complete block design, with four replications. The treatments were applied at 105 and 122 days after harvest of plant cane (DAH) in the TH and TE soil. Ajifer‐8 exhibited higher NH4+–N availability in the 0‐ to 20‐cm soil layer 139 DAH in the TH soil. Ammonium nitrate and CAN resulted in the highest NO3−–N values in the soil within the same period. In the TE soil, the soil mineral N content remained unaltered during all of the sampling dates. The high values of accumulated N uptake by sugarcane in both soils indicates that a good portion of the soil mineral N was take up by the plants. The Ajifer‐8 and CAN fertilizers resulted in higher sugarcane yield in both experiments, and may be used as replacement for conventional N sources. In contrast, the use of ammonium chloride resulted in low values of soil mineral N and accumulated N uptake.

Polyphosphate-based fertilizers are worldwide in use, and their effect on crop yield is often reported to be similar to orthophosphate products, although some studies showed higher yields with polyphosphate applications. However, information on how these fertilizers may influence plant P acquisition is very limited. A pot experiment was carried out under controlled conditions with corn (Zea mays L.) growing on a sandy soil (pH 4.9) and a silty-loam soil (pH 6.9) differing in P-sorption properties. The objective was to evaluate phosphorus fertilizer-use efficiency (PFUE) of several polyphosphate (poly-P) compounds (pyrophosphate [PP], tripolyphosphate [TP], and trimetaphosphate [TMP]) using orthophosphate (OP) as a reference. Focus was put on evaluating plant parameters involved in plant P acquisition, i.e., root length and P uptake per unit of root length. Furthermore, soil P availability was characterized by measuring ortho-P and poly-P concentrations in soil solution as well as in CAL (calcium-acetate-lactate) extracts. The P availability was differentially influenced by the different P sources and the different soils. In the silty-loam soil, the application of poly-P resulted in higher ortho-P concentrations in soil solution. In the same soil, CAL-extractable ortho-P was similar for all P sources, whereas in the sandy soil, this parameter was higher after OP application. In the silty-loam soil, poly-P concentrations were very low in soil solution or in CAL extracts, whereas in the sandy soil, poly-P concentrations were significantly higher. Phosphorus fertilizer-use efficiency was significantly higher for poly-P treatments in the silty-loam soil and were related to a higher root length since no differences in the P uptake per unit of root length among poly-P and OP treatments were found. However, in the sandy soil, no differences in PFUE between OP and poly-P treatments were observed. Therefore, PFUE of poly-P compounds could be explained by better root growth, thereby improving plant P acquisition.

PNSS P04/94PSummaryÐZusammenfassung Applications of polyphosphate-based fertilizers have been reported to have a positive impact on crop yields as compared to orthophosphate sources. Since plants take up P mainly as orthophosphate, hydrolysis rates of polyphosphates into orthophosphates will determine their fertilizer ability. Laboratory and soil incubation experiments were performed to evaluate hydrolysis rates of pyrophosphate (PP), tripolyphosphate (TP), and trimetaphosphate (TMP) in water as well as in two soils having different P-fixing capacities. P availability was characterized by measuring the orthophosphate (ortho-P) and polyphosphate (poly-P) concentration in soil solution as well as the calcium-acetate-lactate (CAL)extractable amounts of both forms. In water, PP was completely hydrolyzed within 15 d, whereas TMP was hydrolyzed only to about 30% after 90 d. In the two soils, polyphosphates hydrolyzed during the incubation period increasing ortho-P concentration in soil solution as well as in CAL extract. At the end of the incubation, no significant differences in ortho-P concentration in soil solution and CAL extract were found in the sandy soil, whereas in the silty-loam soil, polyphosphate applications resulted in higher soil-solution ortho-P concentration. Although polyphosphate hydrolysis is mainly affected by the soil-specific enzymatic activity, it seems that polyphosphates and/or hydrolysis products are preferentially adsorbed/precipitated compared to ortho-P in the silty loam, thereby influencing the P availability from polyphosphate sources.

The potential use of a layered double hydroxide (LDH) to act as a nitrate buffer system in soil in order to reduce the movement of nitrate was investigated. Long-term plant and soil experiments were carried out under greenhouse conditions with the following objectives: (i) evaluate the nitrate adsorption capacity of the LDH during crop growth, and its influence on N uptake, (ii) study the ability of the LDH to adsorb nitrate mineralized during fallow periods, and its influence on nitrate leaching, (iii) evaluate the reversibility for nitrate exchange of the LDH under cultivation conditions, and (iv) determine the nitrate buffer capacity of the soil after LDH application. The LDH adsorbed nitrate from the soil solution during the growth period without affecting plant N uptake. As a result of the adsorption of nitrate on the LDH, the nitrate-N concentration in the soil solution at harvest was reduced by a factor of ten compared to a soil without LDH. The LDH efficiently adsorbed nitrate that was mineralized in the soil during periods without cultivation, reduced nitrate-N leaching losses by about 80%, and kept this nitrate available for a following crop. The nitrate buffer capacity of the soil after 15months increased from 0.3 (without LDH) to 2.7 with the application of 10g LDH kg −1 soil. It is concluded that the LDH has a potential to be used as a long-term nitrate exchanger to control the movement of nitrate in soil, and thereby reduce risks of nitrate leaching in crop production in sensible areas.