Juan Alberto Galantini

The effects of crop rotation and fertilization on wheat productivity in the Pampean semiarid region of Argentina: 2. Nutrient balance, yield and grain quality

Wheat (Triticum aestivum L.) in the semiarid region of Argentina has often been grown as a low-input crop. Rainfall scarcity and distribution are the main characteristics of the region. Consequently, a knowledge of the effect of different management practices is the key to sustainable crop production. The objective of this work was to study the effect of 15 years of different wheat management practices on plant nutrition, dry matter production and grain yield and quality. The treatments were: continuous wheat (WW), wheat-grazing natural grasses (WG) and wheat‐legume: [vetch (Vicia sativa L.) plus oat (Avena sativa L.) or Triticale (Triticum aestivum L. Secale cereale L.)] (WL), with and without fertilizer (Na P) application. The WW and WL treatments involved annual tillage and a long fallow period (4‐6 months) under stubble mulch, and WG involved annual alternate tillage and a short fallow (1 month). The experiment was started in 1975 and the data presented were obtained in 1989. Wheat yields were higher with the WW than with the WG rotation, but in both rotations fertilization was required to obtain better grain quality (protein content higher than 11 per cent). The wheat‐legume rotation resulted in the highest yield, protein content, and better yield components. Fertilizer application did not increase dry matter production but improved nutrient uptake and grain quality. Yield component differences could be attributed to water availability due to different fallow length. The wheat‐legume rotation seemed to be the best practice in the semiarid Pampean region. # 2000 Elsevier Science B.V. All rights reserved.

The effects of crop rotation and fertilization on wheat productivity in the Pampean semiarid region of Argentina. 1. Soil physical and chemical properties

Wheat in the semiarid region of Argentina has often been grown as a low-input crop. Rainfall scarcity and distribution are the main characteristics of this region. The knowledge of the combined effects of crop rotation and fertilization on soil physical and chemical properties are the key for a sustainable crop production. Soil properties for an Entic Haplustoll in the semiarid region of Argentina were evaluated, where different crop rotations were used for 15 years. Wheat‐wheat (Triticum aestivum L.) (WW), wheat‐grazing natural grasses (WG) and wheat‐legume [vetch (Vicia sativa L.) plus oat (Avena sativa L.) or Triticale (Triticum aestivum L. Secale cereale L.)] (WL) rotations with and without fertilizer (64 kg N and 16 kg P ha ˇ1 ) were studied. The annual wheat cropping system (WW) resulted in the lowest soil organic carbon (SOC) and total nitrogen (Nt) levels. Extractable phosphorus (Pext) values were sufficient for wheat growth with all treatments and decreased with depth. Fertilizer applications significantly increased the proportion of large pores (>8.81 mm) in the 0‐0.07 m depth of the WW and WG system plots. A decrease in the proportion of medium size pores (0.19‐8.81 mm) and in the water holding capacity was observed in the WG rotation plots. The fertilized treatments resulted in the following sequence of available water: WL > WW > WG. Bulk density was similar with all treatments for each depth, except with the fertilized WG treatment that had the lowest value in the 0‐0.07 m depth. These results showed the positive influence of legume inclusion (WL) and alternate cattle grazing (WG) on SOC and Nt contents. # 2000 Elsevier Science B.V. All rights reserved.

Bacterial Indicator of Agricultural Management for Soil under No-Till Crop Production

The rise in the world demand for food poses a challenge to our ability to sustain soil fertility and sustainability. The increasing use of no-till agriculture, adopted in many areas of the world as an alternative to conventional farming, may contribute to reduce the erosion of soils and the increase in the soil carbon pool. However, the advantages of no-till agriculture are jeopardized when its use is linked to the expansion of crop monoculture. The aim of this study was to survey bacterial communities to find indicators of soil quality related to contrasting agriculture management in soils under no-till farming. Four sites in production agriculture, with different soil properties, situated across a west-east transect in the most productive region in the Argentinean pampas, were taken as the basis for replication. Working definitions of Good no-till Agricultural Practices (GAP) and Poor no-till Agricultural Practices (PAP) were adopted for two distinct scenarios in terms of crop rotation, fertilization, agrochemicals use and pest control. Non-cultivated soils nearby the agricultural sites were taken as additional control treatments. Tag-encoded pyrosequencing was used to deeply sample the 16S rRNA gene from bacteria residing in soils corresponding to the three treatments at the four locations. Although bacterial communities as a whole appeared to be structured chiefly by a marked biogeographic provincialism, the distribution of a few taxa was shaped as well by environmental conditions related to agricultural management practices. A statistically supported approach was used to define candidates for management-indicator organisms, subsequently validated using quantitative PCR. We suggest that the ratio between the normalized abundance of a selected group of bacteria within the GP1 group of the phylum Acidobacteria and the genus Rubellimicrobium of the Alphaproteobacteria may serve as a potential management-indicator to discriminate between sustainable vs. non-sustainable agricultural practices in the Pampa region.

Long-Term Crop Rotation Effects on Organic Carbon, Nitrogen, and Phosphorus in Haplustoll Soil Fractions

Soil organic matter (SOM) or carbon (SOC) consists of a number of fractions (which can be separated by granulometric wet sieving) having different properties among them. Information on fraction nutrient distribution and long-term crop rotations is lacking for semiarid environments. The objective of this research was to study the agronomic effects on soil OC, N, and P fractions. The humified OC was the largest and least variable fraction of the SOC. Soil under continuous mixed pasture had higher OC contents than under annually tilled treatments. Similarly, soil total nitrogen under the cropped treatments decreased from 1.7 g N kg-¹ in noncultivated soils (reference plots) to 1.0, 0.7 an 0.7 g N kg-¹ under mixed pasture, pasture-crop, and wheat-crop respectively, in the fine soil fraction. The reference plots also showed significantly lower levels of organic phosphorus (P o ) in comparison to the other treatment (from 67.1 w g P o g-¹ to greater than 100 w g P o g-¹ in the fine fraction of the treatments and years). The noncultivated soil showed larger values of P o and inorganic P in the large-size granulometric fraction (0.1-2 mm) than in the soil fine fraction (0-01 mm). However, the rotation treatments had greater concentrations of P in the fine fraction. The P o from the coarse fraction appears to be the most labile and sensitive fraction to tillage and environmental conditions, and may be closely related to P availability.

Continuous Wheat in Semiarid Regions: Long-term Effects on Stock and Quality of Soil Organic Carbon

Abstract Continuous wheat (Triticum aestivum L.) cropping in semiarid regions results in variable dry matter production. As a consequence, the balance of soil organic carbon (SOC) may vary across time. The aim of this research was to assess the dynamics and long-term changes of physically and chemically extracted SOC fractions. Soil samples (0- to 5-, 5- to 10-, and 10- to 20-cm depths) from continuous wheat with (f) and without (nf) fertilizer (N + P) under conventional tillage ((CT) for 25 years) and no-tillage ((NT) for 6 years) were taken during the experiment. Mineral-associated ((MOC) 0–0.053 mm), fine particulate ((POCf) 0.053–0.100 mm), and coarse particulate ((POCc) 0.1–2.0 mm) SOC and humic substances were obtained. The SOC variability depended on water availability during fallow periods (SOC decomposition) or crop cycles (dry matter production). The mean wheat yields were 1.33 (nf) and 2.09 (f) Mg grain ha−1, with an estimated carbon input of 1.64 (nf) and 2.20 (f) Mg C ha−1 year−1. Losses from the initial level were higher in labile fractions, POCc (−75%) and POCf (−53%), than in MOC (−15%). Humic acids present slight differences in their structure and quantity as a result of long-term cropping. Conversion from CT to NT resulted in contrasting results. For an equivalent soil mass, fertilizer application increased SOC by 4.31 Mg ha−1 (under CT) and 7.29 Mg ha−1 (under NT). The SOC turned out to be higher under NT with fertilizer use and lower without application. No-tillage does not increase SOC content by itself; it must be combined with other agricultural practices such as fertilization and/or crop rotation.

Modified Soil‐Test Methods for Extractable Phosphorus in Acidic, Neutral, and Alkaline Soils

Abstract Although numerous soil‐test methods for estimating extractable phosphorus (P) have been developed around the world, their results are difficult to compare because of the very different scale levels used. In the present study, the Bray–Kurtz method (Bray‐P) is used as a reference value. Two other methods [lactate‐P and sodium bicarbonate (NaHCO3)‐P] were modified to facilitate the comparison of extractable‐P determinations, mainly by adjusting the shaking time. These three methods were applied to 101 soil samples from an extensive region of Argentina with soil pH values ranging from 5.5 to 8.5. The results confirm that the Bray‐P and the two modified methods (lactate‐P and NaHCO3‐P) determine similar contents of extractable P but are not applicable to all types of soils and conditions. Equations that minimize the statistical error were selected for soil properties such as organic carbon (OC) content, pH, soluble salts, and calcium carbonate content. Correlation coefficients between Bray‐P and NaHCO3‐P increased to 0.91 and 0.95 in soils with high and low OC levels, respectively. It was also demonstrated that the lactate‐P test is not suitable for soils rich in calcium carbonate or soluble salts. These two modified methods are expected to be useful for testing P values that impact agricultural production.

Soil Phosphorus Dynamics of Wheat-Based Cropping Systems in the Semiarid Region of Argentina

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 µm) and fine (0–100 µm) fractions were analyzed to determine Bray-Kurtz extractable (Pe), organic (Po), inorganic (Pi), and total (Pte) phosphorus. The reference soil was determined at time 0 and compared to a four-year period (time 9 to 12) in three crop sequences: wheat (Triticum aestivum L.)-cattle grazing on natural grasses (WG), continuous wheat (WW), and wheat-legume (WL). Levels of Pe showed differences over time, from 10 to 16 µg g−1 in WG, in line with agriculture and cattle grazing alternate sequences. In WW, P level increased with time, while in WL systems a significant decrease in P from 33.7 to 10.4 µg P g−1 was found during the legume period. Soil P values varied between reference soil and soil samples in year nine and between treatments. Pi was significantly lower in WW, and its concentration increased with time. The coarse fraction of the reference plots had significantly higher levels of Po and Pi than the cultivated treatments, probably a consequence of the particulate organic matter decomposition and coarse mineral particle weathering. The observed changes in Pi content could be attributed to differences in occluded P equilibrium under different soil environments (mainly pH) and crop-tillage-climatic interaction.

Changes in soil pH and phosphorus availability during decomposition of cover crop residues

ABSTRACT The aim of this study was to determine the effect of winter cover crop (CC) residues on soil pH and phosphorus (P) availability. Three incubation assays were performed in pots using two CC: vetch (V) (Vicia villosa Roth.) and oats (Oa) (Avena sativa L.). Soil samples were taken from 10 sites at 0–20-cm depth. The rate of residues were 0 (D0), 10 (D1), 20 (D2), 30 (D3), and 40 (D4) g dry matter kg−1 soil and the soil sampling was after 10, 20, 30, 60, 90, and 120 days of incubation. Soil pH, extractable P (Pe), and soil organic matter (SOM) and its fractions were determined. The pH increase was correlated with the rate applied (D1 < D2 < D3 < D4). No differences were found for pH comparing V and Oa residues with low residue rates. Soil pH changes were dependent from initial pH and SOM fractions in different soils across the incubation period. The multiple regression models showed that the pH changes were dependent on initial pH level and SOM fractions with a high R2 (0.81). CC residues and its quantities produced different changes on pH – especially at the beginning of the incubation – which influenced the P availability.

Soil Humic and Fulvic Acids from Different Land-Use Systems Evaluated By E4/E6 Ratios

ABSTRACT Changes in land-use systems such as the introduction of trees on agricultural land can give rise to changes in the physicochemical properties of the soils, also affecting the quantity and quality of organic matter incorporated into the system. The present study assesses humic substances (HS) in the soil such as humic acid (HA) and fulvic acid (FA) by looking at the relationship between the optic densities determined at 465 and 665 nm (E4/E6 ratio). Topsoil samples (0–20 cm) from pine woodlands 60 years of age were compared with agricultural soils of similar age in the central-south region of the Province of Buenos Aires, Argentina. The pH of the topsoil from beneath the pine trees was highly acidic (5.0 vs. 6.2) and a significant increase in the level of organic carbon (OC) was registered. The carbon to nitrogen (C/N) ratio was also higher (by one order of magnitude) beneath the pine trees, although the humification conditions of the soil organic matter (SOM) were good in the soils of both studied land-use systems. The E4/E6 ratio was higher in the HA and FA 2º (second extraction) beneath the pines, indicating a smaller molecular size of the HS bound to the clay minerals. This fact can be attributed to the higher concentration of hydrogen ions beneath the pines and consequently the loss of polyvalent cations, mainly calcium. The most labile organic molecules (FA 1º – first extraction) were of a larger size in soils beneath the pines, most likely owing to a specific characteristic of the Pinus genus, although the fraction in question constitutes a minority fraction among the HS. Clear differences were established between the E4/E6 ratios in HA and FA, making this a highly useful method for determining molecular changes in HS as a result of changes in land use.

A Rapid Chemical Method for Estimating Potentially Mineralizable and Particulate Organic Nitrogen in Mollisols

ABSTRACT The objective of this study was to obtain an indicator of labile nitrogen (N) through a cost- and time-saving procedure by evaluating the relationships among potentially mineralizable N (N0), particulate organic matter N (POM-N) and soil organic N extracted through partial soil digestion with different concentrations of sulfuric acid (H2SO4). Soil sampling (0–20 cm) was from nine fields under no-tillage. The N0 and POM-N were determined by long-term aerobic incubation and soil physical fractionation, respectively. A simple chemical method was developed by soil digestion at 100°C for 4 h with different concentrations of H2SO4 (0.1, 0.5, 1, 6, 12, and 24 mol L−1). All acid concentrations showed significant relationships with N0 as POM-N; however, the best prediction was resulted for 0.5 mol L−1 (R2 = 0.90–0.94, respectively), thus using this methodology as soil labile N indicator. This method would optimize N0 and POM-N estimation in short term and at a low cost.