The dynamics of soil P forms and particle size fractions was studied under three wheat-based cropping sequences in production systems of Argentina. The whole soil and its coarse (100–2000
Plants absorb mainly inorganic phosphorus (Pi) but the organic phosphorus (Po) is also an important reservoir for plant nutrition [
Several soil studies have shown P deficiencies in the semiarid and semihumid regions of Argentina, and that extractable P content is lower due to soil pedogenetic characteristics and the agricultural history of the region [
McKenzie et al. [
The P compounds strongly bound to soil fine fraction have been shown to be unaffected by tillage treatments [
When organic and inorganic phosphorus are determined, in both fine and coarse fractions, it will expect that Po content in the coarse fraction would be the most sensitive to crop-tillage management changes. This Po is associated with particulate organic matter, a labile SOM fraction. However, Pi content in the coarse fraction would be more stable because of its relationship with sand size. On the other hand, Po content in the fine fraction would be stable because of its relationship with humified organic materials, while the Pi content would be more strongly linked to plant P availability due to its relationship with fine size minerals of soil.
We hypothesized that crop sequences can modify the P distribution within different soil particle sizes, affecting its availability for plants. The main objective of this study was to evaluate the changes caused by different wheat-based cropping sequences on the distribution of P forms in particle size fractions in an entic Haplustoll of the semiarid Pampas.
This study was carried out at the Agricultural Experimental Research Station of INTA, Bordenave (63° 01′ 20′′ W and 37° 51′ 55′′ S), Province of Buenos Aires, Argentina. The climate in this area is temperate (continental moderated), with a mean annual temperature of 15°C. Mean annual rainfall is about 667 mm (1928–2013), concentrated in autumn and spring, with a dry period at the end of the winter and semidry period in the middle of summer. During the studied period mean annual precipitation was around 900 mm. Mean annual evapotranspiration rate is 28% more than the climatic offer. The main soil subgroup is entic Haplustoll (FAO: Haplic Kastanozem), which is a thermal, sandy loam, typically of this region, which has low to medium fertility and is sensitive to wind erosion, with a calcareous layer located between 0.8 and 1.0 m in depth [ WG, one year wheat and one year cattle grazing on natural grasses ( WW, continuous wheat. Following harvest, there was a 4–6-month fallow (January–June) under stubble mulch used for soil moisture storage, mechanical weed control using chisel ploughing to a depth of 0.20 m preceded wheat deep-furrow seeding, which deposits the seed 8–10 cm deep. WL, 3 years of wheat and 3 years of red clover (
Crop sequence for each cropping system is shown in Table
Crop sequences in the studied production systems and mean annual rainfall.
Years from the beginning | WG | WW | WL | Annual rainfall (mm) |
---|---|---|---|---|
0 | ||||
1 | Grass/oat | Wheat | Wheat | 683.7 |
2 | Wheat | Wheat | Wheat | 998.1 |
3 | Grass/oat | Wheat | Wheat | 965 |
4 | Wheat | Wheat | Red clover | 611.9 |
5 | Grass | Wheat | Red clover | 781.8 |
6 | Wheat | Wheat | Red clover | 647.1 |
7 | Grass | Wheat | Wheat | 697.1 |
8 | Wheat | Wheat | Wheat | 612 |
9 | Grass | Wheat | Wheat | 1194.7 |
10 | Wheat | Wheat | Red clover | 1108.3 |
11 | Grass | Wheat | Red clover | 678.9 |
12 | Wheat | Wheat | Red clover | 641.6 |
Wheat (
Surface soil samples (0–15 cm depth) were obtained after wheat growing cycle during November of each sampling year; 3 subsampling points were located using GPS for subsequent sampling. Three blocks were randomly located in the three production systems and in reference soil; three composite samples consisting of three soil sample each were obtained from each block. Soil texture differences were not observed between different blocks or treatments, with mean values for clay and silt of 101 and 218 g kg−1, respectively. The samples were returned to the lab, air dried, sieved (<2 mm), and stored for analysis. Soil was sampled 9 years after initiating crop rotations and the subsequent years.
Soil pH with a glass electrode at a 1 : 2.5 water ratio [ extractable (Pe) by Bray-Kurtz 1 [ total extractable (Pte), [ total (Pt) with sodium carbonate [ and organic (Po) and inorganic (Pi) by Saunders and Williams [
Inorganic P in all extracts was determined by the ammonium vanadate colorimetric method [
For the size fractionation of soil, we used the wet sieving of soil [
Statistical analysiswas performed using analysis of the variance (ANOVA) and InfoStat’s least significant differences (LSD) procedure [
Comparisons among years were not made due to significant interactions over time, probably as a consequence of the crop-rainfall interaction. For that reason, the statistical analysis was performed in two ways: (1) by comparing the Ref soil with each treatment in year 9 and (2) by comparing the three treatments in each year (9, 10, 11, and 12 years).
Soil organic and inorganic P concentrations decreased under the different cropping sequences with a sharp decline in the Po : (Po + Pi) ratio. Inorganic P was the most abundant form in both the reference and cultivated soils (Table
Soil organic carbon (SOC), organic (Po), inorganic (Pi), total extractable (Pte), total and extractable phosphorus (Pe), Po : (Po + Pi) relationships, and pH of different cropping systems (CS).
Year | CS | SOC | Po | Pi | Po : (Po + Pi) | Pte | Pt | Pe | pH |
---|---|---|---|---|---|---|---|---|---|
% | mg kg−1 | mg kg−1 | |||||||
0 | Ref | 1.28 | 189.0 | 328.0 | 0.37 | 529.0 | 562.1 | 30.9 | 6.7 |
9 | WG | 1.13a* | 77.7a** | 294.7a* | 0.20a** | 289.6b** | 412.2b* | 14.3b** | 6.8a ns |
WW | 0.89b** | 50.4b** | 172.0b** | 0.22a** | 230.0b** | 396.3b* | 11.7b** | 6.6b ns | |
WL | 1.14a* | 82.2a** | 290.5a* | 0.22a** | 417.3a* | 456.3a* | 33.7a ns | 6.6b ns | |
10 | WG | 1.08a | 133.7a | 231.1a | 0.37a | 396.7a | 418.0a | 10.2b | 6.4ab |
WW | 0.84b | 75.5b | 159.0b | 0.32b | 344.1b | 378.2ab | 13.1b | 6.3b | |
WL | 1.11a | 107.3ab | 230.5a | 0.32b | 339.7b | 362.1b | 21.2a | 6.5a | |
11 | WG | 1.07a | 86.7 | 217.5b | 0.38a | 350.2a | 440.0a | 15.7b | 6.3a |
WW | 0.85b | 84.0 | 176.3c | 0.32b | 316.8b | 407.2a | 18.8b | 6.3a | |
WL | 1.12a | 81.5 | 245.7a | 0.33b | 289.7b | 328.4b | 26.0a | 6.1b | |
12 | WG | 1.16a | 85.4 | 218.1b | 0.28a | 343.0a | 420.0a | 11.6b | 6.4a |
WW | 0.95b | 65.7 | 233.7ab | 0.22b | 365.2a | 411.3a | 27.2a | 6.1b | |
WL | 1.27a | 78.3 | 250.2a | 0.24b | 280.1b | 321.6b | 10.4b | 6.2b |
After years with different cropping systems, the Po in the fine fraction was stable among treatments throughout sampling years (Table
Soil organic (Po), inorganic (Pi), and total extractable phosphorus (Pte) in the fine and coarse fraction of different cropping systems (CS).
Year | CS | Fine fraction | Coarse fraction | ||||
---|---|---|---|---|---|---|---|
mg kg−1 | mg kg−1 | ||||||
Po | Pi | Pte | Po | Pi | Pte | ||
0 | Ref | 60.0 | 178.0 | 329.0 | 129.0 | 149.0 | 199.0 |
9 | WG | 72.1b ns | 199.8a* | 221.5b** | 5.6a** | 95.2a** | 98.1a** |
WW | 50.0c ns | 98.4b** | 149.7c** | 2.1a** | 73.6b** | 80.2b** | |
WL | 88.1a* | 200.8a* | 331.4a ns | 1.2a** | 89.7a** | 85.9b** | |
10 | WG | 127.8a | 149.6a | 305.7a | 6.1a | 81.5a | 91.0a |
WW | 72.2b | 93.8b | 256.3b | 3.3a | 65.2b | 87.8a | |
WL | 101.6a | 144.3a | 263.2ab | 5.7a | 86.2a | 76.5b | |
11 | WG | 75.4a | 143.4a | 264.0a | 17.5a | 74.1b | 86.2a |
WW | 75.4a | 121.0b | 239.5ab | 6.8a | 55.3c | 77.3b | |
WL | 77.6a | 151.5a | 207.9b | 8.9a | 94.1a | 81.8ab | |
12 | WG | 84.8a | 133.9b | 259.2a | 1.6a | 84.2a | 83.8a |
WW | 57.9b | 150.7ab | 256.6ab | 8.6a | 83.0a | 108.5b | |
WL | 89.3a | 161.6a | 199.0b | 4.3a | 88.7a | 81.1a |
The most important changes in Po and Pi contents in coarse fractions were found between the reference soil and the treatments, mainly due to the Po content depletion (Table
The main differences in P fraction were observed between reference and cultivated soils. The highest variability of Po with respect to SOC suggests quality variations during crop sequences [
The effect of continuous wheat with fertilizer application on SOC content was a consequence of annual tillage and low residue input. A grassing period without tillage (WG and WL) and an increased residue input due to biological fixed N (WL) could explain the higher SOC in WG and WL as compared with WW [
During the 4-year study period, treatments did not affect the dynamics of the organic P fraction. Other studies on Po in Pampas soils detected slight differences between management systems [
Considering that the Po plus Pi quantity ranged between 222 and 373 mg kg−1 and the Pte ranged from 230 to 417 mg kg−1, a variable fraction of the soil P is not quantified by the Saunders and Williams [
After ploughing pasture soils, available P increased over the plant requirements suggesting that physicochemical equilibrium could result in precipitation of inorganic P forms such as apatite (Ca5(PO4)3(OH,F,Cl)) or brushite (CaHPO4
In the WG treatment, Pe content ranged from 10 to 16 mg kg−1 over time, showing higher values during grassing than wheat years. In continuous wheat, Pe level increased over time, whereas in WL they decreased significantly, from 33.7 mg kg−1 at the end of the wheat period to 10.4 mg kg−1 at the end of the legume period. A decrease during legume periods was also observed by other authors [
The inherent variability of Po, due to different plant residues and analysis method variability of this organic fraction, due to different plant residues and analysis method variability (obtained by difference between Pi and Po plus Pi), may explain the difficulty in detecting expected differences among sequences. Nine years under WW produced the lowest Pi content in the fine fraction, but no changes were found in WG and WL cropping systems. During the following sampling times, Pi concentrations tended to increase in WW and to decrease in WG and WL treatments. The importance of the observed differences would suggest that it might be a consequence of the type and accessibility of P forms. This observation could be accounted for by the lower variability of Pte values. This increase of Pi in WW might occur because of the change from inorganic occluded or nondetected P to inorganic available forms, and later quantified in the analysis. In addition, losses due to plant uptake could be compensated with P from the coarse fraction.
More than 95% of the organic and 35% of the inorganic P as compared to the reference soil was lost in the coarse fraction due to cultivation. Probably, there was a combined effect of crop sequences (due to P requirement and residue input) and climatic condition effects on crop production and fresh organic matter mineralization [
The effect of cropping system on P content in whole soil and its fine and coarse size fractions in a semiarid Haplustoll can be summarized as follows: The major differences for all P forms were observed when cropping systems were compared with the reference soil. Reduction in the Po : (Po + Pi) ratio was mainly due to decomposition of SOM and this decomposition rate was dependent on moisture availability. The Pe content was modified by cropping system, increasing under a WW sequence and decreasing under WL or WG. The main effect of a cropping system on P dynamics was the rapid decomposition of the P in the soil coarse fraction with an increase of Pi in the fine fraction. Soil P distribution depended mainly upon tillage effect on SOM mineralization and particle size distribution, as well as the cropping system and the potential changes in water availability modified physicochemical equilibrium.
Soil cropping modifies P distribution within different soil particle sizes. A decrease of the P in coarse fraction (both organic and inorganic forms) was observed. As was hypothesized, crop sequences modify P distribution on soil particle size. The Pi form is mainly affected.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to thank the institutions which provided the funding and personnel for this research to the “Comisión de Investigaciones Científicas” (CIC, Pcia. Bs. As.) and the “Estación Experimental Agropecuaria Bordenave INTA”.