The effects of 25 years of annual applications of P fertilizer on the accumulation and migration of soil Olsen‐P, and the effects of soil residual P on crop yields by withholding P application for the following 5 years, were evaluated in a subtropical region. Annual application of P fertilizer for 25 years to crops in summer (groundnut), winter (wheat, mustard or rapeseed) or in both seasons raised the Olsen‐P status of the plough layer (0–15 cm) from initially very low (12 kg P ha−1) to medium (18 kg P ha−1) and very high levels (40–59 kg P ha−1), depending on the amount of P surplus (amount of fertilizer applied in excess of removal by crops) (r = 0.86, P ≥ 0.01). However, only 4–9% of the applied P fertilizer accumulated as Olsen‐P to a depth of 15 cm (an increase of 2 mg kg−1per 100 kg ha−1 surplus P) in the sandy loam soil. In the following 5 years, the raising of 10 crops without P fertilizer applications decreased the accumulated Olsen‐P by only 20–30% depending upon the amount of accumulated P and crop requirements. After 29 years, 45–256 kg of residual P fertilizer had accumulated as Olsen‐P ha−1 in the uppermost 150 cm with 43–58% below 60 cm depth; this indicates enormous movement of applied P to deeper layers in this coarse textured soil with low P retention capacity for nutrients. Groundnut was more efficient in utilizing residual P than rapeseed; however, for both crops the yield advantage of residual P could be compensated for by fresh P applications. These results demonstrated little agronomic advantage above approximately 20 mg kg−1 Olsen‐P build‐up and suggested that further elevation of soil P status would only increase the risk of environmental problems associated with the loss of P from agricultural soils in this region.

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A pot experiment was conducted in a growth chamber to investigate the effects of dandelion (Taraxacum officinale) and quackgrass (Elymus repens) on the growth of hybrid poplar (Populus deltoides×Populus×petrowskyana var. Walker). Single hybrid poplar seedlings were grown in pots either alone (SHP) or with four or eight dandelion plants per pot or with one or three quackgrass plants per pot in two soils collected from sites previously managed for alfalfa and pasture near Meadow Lake, Saskatchewan, Canada. Hybrid poplar and weed species were harvested approximately 7 and 14 weeks after planting. Approximately 14 weeks after planting, hybrid poplar shoot biomass in the SHP treatment was 28 g for the pasture soil and 22 g for the alfalfa soil. Corresponding shoot biomass for hybrid poplar grown with the dandelion and quackgrass treatments varied from 0.54 to 0.81 g and 0.3 to 3.66 g, respectively. Other hybrid poplar growth parameters including stem height, root collar diameter and fresh root biomass were similarly reduced by competition with the weed species. During the growing period, soil solution N and K concentrations decreased several-fold in both soils; however, the magnitude of decrease was comparatively higher in the weed treatments. Nitrogen, P and K uptake by hybrid poplar was greater in the SHP treatment in both the soils (337-425, 38-49 and 396-463 mg pot −1 , respectively) compared to the weed treatments (4-28, 0.4-6.2 and 0.6-54.0 mg pot −1 , respectively) by the end of the experiment. The presence of quackgrass and dandelion severely affects the growth of hybrid poplar by causing intense below-ground competition for nutrients.

The substitution of the widely practiced crop‐residue burning by residue incorporation in the subtropical zone requires a better understanding of factors determining nutrient mineralization. We examined the effect of three temperature (15°C, 30°C, and 45°C) and two moisture regimes (60% and 90% water‐filled pore space (WFPS)) on the mineralization‐immobilization of N, P, and S from groundnut (Arachis hypogae) and rapeseed (Brassica napus) residues (4 t ha–1) in two soils with contrasting P fertility. Crop‐residue mineralization was differentially affected by incubation temperature, soil aeration status, and residue quality. Only the application of groundnut residues (low C : nutrient ratios) resulted in a positive net N and P mineralization within 30 days of incubation, while net N and P immobilization was observed with rapeseed residues. Highest N and P mineralization and lowest N and P immobilization occurred at 45°C under nearly saturated soil conditions. Especially net P mineralization was significantly higher in nearly saturated than in aerobic soils. In contrast, S mineralization was more from rapeseed than from groundnut residues and higher in aerobic than in nearly saturated soil. The initial soil P content influenced the mineralization of N and P, which was significantly higher in the soil with a high initial P fertility (18 mg P (kg soil)–1) than in the soil with low P status (8 mg P (kg soil)–1). Residue‐S mineralization was not affected by soil P fertility. The findings suggest that climatic conditions (temperature and rainfall‐induced changes in soil aeration status) and residue quality determine N‐ and S‐mineralization rates, while the initial soil P content affects the mineralization of added residue N and P. While the application of high‐quality groundnut residues is likely to improve the N supply to a subsequent summer crop (high temperature) under aerobic and the P supply under anaerobic soil condition, low‐quality residues (rapeseed) may show short‐term benefits only for the S nutrition of a following crop grown in aerobic soil.

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