No abstract

No abstract

The effect of tillage systems and crop rotation on microbial biomass phosphorus (MBP) and acid phosphatase (P‐ase) activity, and the amount of different phosphorus (P) forms measured by 31P‐NMR spectroscopy were studied on a field experiment carried out in a temperate Ultisol from southern Chile. Two tillage systems, no tillage (NT) and conventional tillage (CT) and two crop rotations, oat–wheat (OW) and lupine–wheat (LW) were evaluated 4 yr after the start of the experiment to determine the effects of such management on some soil biological parameters and P forms at three depths (0–5, 0–10 and 10–20 cm). Microbial biomass P ranged from 6.5 to 22.6 mg/kg, whereas the mean total P (PT) was 1995 mg/kg for all treatments (OW and LW). Microbial biomass carbon (MBC) and surface P accumulation (at 0–5 cm depth), including Olsen P, MBP, orthophosphate monoesters (monoester‐P), were larger under NT than CT. Tillage effects were greater than crop rotation effects in enhancing P availability. The LW rotation showed enhanced P‐ase activity and increased monoester‐P forms (57 vs. 30% of the total integral area of the spectra, in average) compared with OW. Nevertheless, OW rotation increased orthophosphate (ortho‐P), especially at 10–20 cm. Microbial biomass carbon ranged from 532 to 2351 mg/kg, which represented 1.2–4.5% of total organic C (Co). Furthermore, MBP correlated positively with MBC (r = 0.80), Olsen P (r = 0.77), Co (r = 0.77), pH (r = 0.65), PT (r = 0.65) and P‐ase activity (r = 0.57), suggesting the importance of the microbial biomass on soil P availability.

No abstract

Volcanic ash derived soils represent between 50-60% of the total arable land area of southern of Chile, and they are the most important soils for pasture production. In these soils, high phosphorus (P) fixation and, in turn, low P availability and high aluminium (Al) soluble concentrations (at low pH) are the most limiting factors for pasture production. At the same time, the complexes between Al-or iron-(Fe) and organic matter as well as short-range order alumino-silicates (allophane) allow the retention of huge quantities of soil P. The aim of this work was to assess the status of P by both sequential extraction procedure (Hedley) and 31 P-NMR analysis as influenced by Al and Fe in volcanic grasslands Andisols (Pemehue, Gorbea, Piedras Negras and Llastuco Soil Series) from Southern Chile. We applied Hedley chemical sequential fractionation to soils in order to examine the potential differences in extractable soil inorganic P (P i ) and organic P (P o ) fractions. We also determined total P and Olsen P in these grassland Andisols. Oxalate and pyrophosphate were employed to determine the active and organic matter complexed Al and Fe, respectively. Furthermore, we quantified Al and Fe in extracts of the Hedley P fractions. We found that Al extracted in oxalate was correlated positively with labile P o concentration, specifically with both the NaHCO 3 -P o (r=0.45, P≤0.01), and the NaOH-P o (r=0.43, P≤0.01) fractions. This observation was reinforced by 31 P-NMR analysis that showed higher monoester P and myo-IP 6 content in soils with higher amounts of oxalate Al. Hedley sequential fractionation procedure confirmed the role of Al in the NaOH-P o fraction for promoting P o storage, as both fractions were correlated (r=0.33, P≤0.05). In addition, Fe plays a substantial role in recalcitrant P accumulation as we found a high correlation between residual P and oxalate Fe (r=0.55, P≤0.01).

The aim of this study was to determine if three cereal crops differed in their behavior to take up soil and fertilizer P, with emphasis on the relationship between phosphatase activity and P fractionation. We used a vertical rhizobox experiment with wheat, oat, and barley sown on Chilean Andisol (Barros Arana Series) with low P availability under greenhouse conditions. Plants were fertilized with the equivalent of 100 kg P ha −1 of triple superphosphate (TSP) or rock phosphate (RP). Plant biomass was determined for each of the three cereal plant species. Additionally, phosphatase (P-ase) activity in roots, soil in presence of roots (soil+R), and soil in absence of roots (soil−R) after 60-day growth were evaluated, and soil P fractionation was determined using the Hedley procedure. Fertilizer increased both P uptake and biomass production, particularly in shoots. The P uptake efficiency (ΔP uptake between fertilized and unfertilized treatment/P input) was low (4.6%) and similar for both fertilizers for oat, but RP was more efficient for wheat (> 30%) and even more so for barley (nearly threefold), due the higher shoot P concentration of RP fertilized plants, which could be attributable to a major P-ase activity in plants fertilized with RP. Despite, fertilizer P was most clearly identified in labile inorganic soil fractions with Olsen P being greater after TSP addition than RP. In particular, plants showed contrasting soil+R P-ase activity inducing differences in soil+R P speciation, increasing labile NaHCO 3-P i and NaOH-P i fractions with TSP. Strong relationships were found between the sum of labile P i fractions and P uptake. We conclude that slower release of RP has a positive impact on P-ase activity and leads to better fertilizer efficiency than TSP, especially for barley.

The aim of this study was to compare P bioavailability in a Nothofagus rainforest Andisol (FS) and an adjacent clear-cut grassland soil (GS) in southern Chile to evaluate the effects of land use change on P chemical forms determined by chemical fractionation and 31 P-NMR spectroscopy. Total phosphorus (P), Olsen P, microbial P, different soil P fractions (determined using a modified Hedley procedure), 31 P-NMR spectroscopy results, acid phosphatase (P-ase) activity, pH and organic C were analyzed and compared. Forest samples were collected from the mineral soil at a depth of 2-20 cm and were compared with those collected from grassland soil at the same depth. Total P ranged from 2028 mg kg -1 (FS) to 2157 mg kg -1 (GL) and total organic P ranged from 829 mg kg -1 (FS) to 1176 mg kg -1 (GL). On the contrary, Olsen P, microbial P, labile P and P-ase activity were higher in the evergreen forest soil than in the grassland, with the predominance of the moderately labile (NaOH-P o ) fraction, which ranged from 668 to 720 mg kg -1 in both soils. Phosphorus was mainly present in monoester-P form in the NMR extract in both soils (67 % on average). Other 31 P-NMR signals were identified as C 2 -myoinositol phosphate and scyllo-inositol hexakisphosphate. The results suggest that land use change from forest to grassland will reduce P bioavailability and P-ase activity.

No abstract