Tillage system and P fertilizer placement can aff ect plant root growth and therefore water and nutrient uptake. Th e objective of this study was to evaluate the eff ect of P fertilizer placement and tillage system on soybean [Glycine max (L.)] root growth and grain yield under induced drought stress. A fi eld study was performed at two locations in southern Brazil, during the 2014/2015 season. Phosphorus fertilizer placement and tillage combinations were evaluated using triple superphosphate at 31 kg P ha-1. Treatments included: (i) strip-tillage with deep band (ST-DB); (ii) strip-tillage with band-applied 5 by 5 cm (ST-B); (iii) no-till with broadcast (NT-BR); (iv) no-till with band-applied 5 by 5 cm (NT-B); and (v) no-till with surfaceband (NT-SB). Root length density (RLD) and root diameter were evaluated at 0-to 25-cm depth in 5-cm intervals. Yield was evaluated under rainfed as well as under induced drought conditions. Th e ST-DB treatment showed increased total RLD among treatments, with about 58% greater RLD than the NT-BR treatment, and 46% greater RLD than the NT-B treatment at the 15-to 25-cm soil depth. Furthermore, the soybean yield penalty with the ST-DB treatment was lower than any other treatment with a yield reduction of about 9 and 0.3% at respective locations under induced drought stress. Results from our study showed that the ST-DB treatment contributed to enhance soybean root growth at deeper soil layers and improved overall resilience to induced drought.

Environmental concerns have stimulated increased interest in NO−3 accumulation in soils. The aeration status of the soil, which is mainly governed by the water content and temperature, is a central factor. The biological process responsible for NO−3 accumulation, nitrification, was measured to estimate the combined effects of water content and temperature and determine their joint effect on soil aeration. The effects of temperatures of 15, 20, 25, 30, and 35°C and water contents equivalent to 0.35, 0.42, 0.50, 0.52, 0.57, and 0.60 relative water content (volumetric water content/total porosity) on the nitrification activity of soil samples containing 2‐mm sieved soils taken at 0 to 20 or 20 to 40‐cm depth were determined by measuring NO−3 accumulation for 17 h. A descriptive model including three biological parameters, maximum nitrification rate (Nrmax), optimal relative water content (Θopt), and temperature, was developed. Maximum Nrmax occurred at 25.5°C in 0‐ to 20‐cm soil and at 20°C in 20‐ to 40‐cm soil, suggesting an adaptation of soil nitrifying populations to the temperature regime of the soil. The Nrmax value was negatively related to Θopt, and Θopt was dependent on temperature (T). This Θopt (T) relationship was parabolic in nature, with Θopt being at a minimum between 20 and 25°C. It could be simulated using O2 diffusion and respiration rates, inferring that these processes influenced Θopt and T correlation. The ranges of O2 concentrations favorable to maximum nitrification within an aggregate volume fraction were estimated for different temperatures. Nitrification was generally maximum when the intraaggregate pore spaces were saturated with water, with no water in the interaggregate pore space (i.e., 0.44 relative water content at 25°C and 0.36 at 20°C at 0‐ to 20‐ and 20‐ to 40‐cm depths).

Phosphorus fertilizer placement can have signifi cant agronomic and environmental implications in long-term no-till (NT) systems. Th e objective of this study was to evaluate soybean [Glycine max (L.) Merr.] response to P fertilizer placement strategies under long-term NT management. A fi eld study was performed near Nao-Me-Toque-RS (Location 1) and Sao Sepe-RS (Location 2), southern Brazil, during the 2014/2015 growing season. Th e experimental design was a randomized complete block with three replications. Triple superphosphate was applied using fi ve strategies: (i) strip tillage with deep band (ST-DB); (ii) strip tillage with band-applied 5 by 5 cm (ST-B); (iii) no-till with broadcast (NT-BR); (iv) no-till with band-applied 5 by 5 cm (NT-B); (v) and no-till with surface band (NT-SB). Plant height, dry weight, and P uptake were evaluated at 20, 40, 60, and 80 d aft er emergence (DAE) as well as P removed and grain yield at harvest. Th e ST-B application promoted greater plant height, dry weight, and P uptake at 80 DAE. However, ST-DB showed the greatest P removal compared to other treatments. Also, greater yields were obtained for ST-DB and NT-BR. Soil sampling aft er harvest showed that ST-DB increased soil test P levels by 19 and 11% at the 15-to 25-cm layer for Locations 1 and 2, respectively. While NT-BR increased soil test P by 43 and 36% at the 0-to 5-cm layer for Locations 1 and 2, respectively. Deep band P fertilizer placement maintained or increased soybean yield and P use under long-term NT in tropical soils.

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