Carina R. Alvarez

Mechanisms for the increase in phosphorus uptake of waterlogged plants: soil phosphorus availability, root morphology and uptake kinetics

Abstract Waterlogging frequently reduces plant biomass allocation to roots. This response may result in a variety of alterations in mineral nutrition, which range from a proportional lowering of whole-plant nutrient concentration as a result of unchanged uptake per unit of root biomass, to a maintenance of nutrient concentration by means of an increase in uptake per unit of root biomass. The first objective of this paper was to test these two alternative hypothetical responses. In a pot experiment, we evaluated how plant P concentration of Paspalum dilatatum, (a waterlogging-tolerant grass from the Flooding Pampa, Argentina) was affected by waterlogging and P supply and how this related to changes in root-shoot ratio. Under both soil P levels waterlogging reduced root-shoot ratios, but did not reduce P concentration. Thus, uptake of P per unit of root biomass increased under waterlogging. Our second objective was to test three non-exclusive hypotheses about potential mechanisms for this increase in P uptake. We hypothesized that the greater P uptake per unit of root biomass was a consequence of: (1) an increase in soil P availability induced by waterlogging; (2) a change in root morphology, and/or (3) an increase in the intrinsic uptake capacity of each unit of root biomass. To test these hypotheses we evaluated (1) changes in P availability induced by waterlogging; (2) specific root length of waterlogged and control plants, and (3) P uptake kinetics in excised roots from waterlogged and control plants. The results supported the three hypotheses. Soil P avail-ability was higher during waterlogging periods, roots of waterlogged plants showed a morphology more favorable to nutrient uptake (finer roots) and these roots showed a higher physiological capacity to absorb P. The results suggest that both soil and plant mechanisms contributed to compensate, in terms of P nutrition, for the reduction in allocation to root growth. The rapid transformation of the P uptake system is likely an advantage for plants inhabiting frequently flooded environments with low P fertility, like the Flooding Pampa. This advantage would be one of the reasons for the increased relative abundance of P. dilatatum in the community after waterlogging periods.

Short-term effects of tillage systems on active soil microbial biomass

Abstract Conservation tillage, and especially no-tillage, induce changes in the distribution of organic pools in the soil profile. In long-term field experiments, marked stratification of the total soil microbial biomass and its activity have been observed as consequence of the application of no-tillage to previously tilled soils. Our objective was to study the evolution of the total and active soil microbial biomass and mineralized C in vitro during the first crop after the introduction of no-tillage to an agricultural soil. The experiment was performed on a Typic Hapludoll from the Argentinean Pampa. Remaining plant residues, total and active microbial biomass and mineralized C were determined at 0–5 cm and 5–15 cm depths, at three sampling times: wheat tilling, silking and maturity. The introduction of no-tillage produced an accumulation of plant residues in the soil surface layer (0–5 cm), showing stratification with depth at all sampling dates. Active microbial biomass and C mineralization were higher under no-tillage than under conventional tillage in the top 5 cm of the profile. The total soil microbial biomass did not differ between treatments. The active soil biomass was highly and positive correlated with plant residues (r2=0.617;P<0.01) and with mineralized C (r2=0.732;P<0.01). Consequently, the active microbial biomass and mineralized C reflected immediately the changes in residue management, whereas the total microbial biomass seemed not to be an early indicator of the introduction of a new form of soil management in our experiment.

PREDICTIONS OF AVAILABLE NITROGEN CONTENT IN SOIL PROFILE DEPTH USING AVAILABLE NITROGEN CONCENTRATION IN SURFACE LAYER

The Rolling and Flat Pampas are vast plains which contribute with more than half of the Argentine wheat production. In the 80s three extensive studies were preformed in these regions, which determined nitrate content in the 0–20, 20–40, and 40–60 cm soil layers, comprising a wide range of soil, climatic and cultural variables. More recently, we determined the mineral-N (N) content at 0–30 and 30–60 cm depths, in a set of farmers plots under different tillage systems and previous crop. These datasets were used in order to establish simple equations to predict N availability in the 0–60 cm stratum, from the N availability in the upper layer. Nitrate content at wheat presowing time presented a clear stratification pattern with depth. For the fist dataset, strong relationships were obtained between the nitrate content at different soil depths, with previous crops of corn, sunflower or sorghum (R2=0.52 to 0.90, P<0.05), but low adjustments were obtained for soybean. In the second set of data, mineral-N measured in the 0–30 and 30–60 cm layers were also linearly correlated, for each tillage treatment (plow, disk and no-till) and previous crop (R2>0.62). Two general equations were obtained that relates N availability in the upper layer with the availability in the 0–60 cm layer (R2=0.89, P<0.001), comprising a wide range of management and environmental conditions. As N fertilizer recommendations in these Pampean Regions required soil nitrate content to a depth of 60 cm, close predictions of N availability in the this stratum from the upper layer can facilitate sampling and extent the use of the diagnosis methodologies by farmers.

Association between soil organic matter and wheat yield in Humid Pampa of Argentina

The Rolling and Flat Pampas are vast plains located in Argentina. Wheat is one of the most economically important crops of these regions. Two extensive studies have been preformed to evaluate the effects of nitrogen fertilization on wheat yield at field scale. We pooled these published data to establish relationships between wheat yield and soil and climate variables under a wide range of soils and management conditions. Total soil carbon was the variable more associated with yield (r2=0.25). The increase in grain production expected between soils with low and high carbon levels rounded 2,200 kg ha− 1. A multivariate model which included carbon in the light fraction, potential mineralizaable nitrogen, available mineral nitrogen, and rainfall was obtained, explaining 50% of wheat yield variability. These results highlight the importance of organic matter on grain production in the Humid Pampas. This effect can be due to the role of organic soil components as source of nutrients for crops.

Assessment of topsoil properties in integrated crop–livestock and continuous cropping systems under zero tillage

A regional study was conducted in the northern Pampas of Argentina in order to compare soil quality at proximal cropland sites that are managed under either continuous cropping (CC) (n = 11) or integrated crop–livestock (ICL) (n = 11) systems under zero tillage. In the ICL system, samples were taken in the middle of the agricultural period. Although soil total and resistant organic carbon (TOC, ROC) were significantly higher in silt loam soils than in loam/sandy loam soils, variations in carbon concentration were not associated with differences in soil management. Soil relative compaction was the only property that was significantly (P < 0.05) affected by the soil type × management interaction. Soil relative compaction values were significantly lower with ICL in loam/sandy loam soils, but there were no significant differences in silt loam soils. Structural instability index showed little change from CC to ICL sites, indicating that there was no soil structural damage. Soil penetration resistance was significantly higher in ICL soils within the first 0.075 m of soil depth, slightly exceeding the critical threshold (2000 kPa). However, firmer topsoil under ICL was not due to shallow compaction, as evidenced by no increase in soil bulk density.

ROOT ABUNDANCE OF MAIZE IN CONVENTIONALLY- TILLED AND ZERO-TILLED SOILS OF ARGENTINA (1)

SUMMARY Maize root growth is negatively affected by compacted layers in the surface (e.g. agricultural traffic) and subsoil layers (e.g. claypans). Both kinds of soil mechanical impedances often coexist in maize fields, but the combined effects on root growth have seldom been studied. Soil physical properties and maize root abundance were determined in three different soils of the Rolling Pampa of Argentina, in conventionally-tilled (CT) and zero-tilled (ZT) fields cultivated with maize. In the soil with a light Bt horizon (loamy Typic Argiudoll, Chivilcoy site), induced plough pans were detected in CT plots at a depth of 0–0.12 m through significant increases in bulk density (1.15 to 1.27 Mg m -3 ) and cone (tip angle of 60 o) penetrometer resistance (7.18 to 9.37 MPa in summer from ZT to CT, respectively). This caused a reduction in maize root abundance of 40– 80 % in CT compared to ZT plots below the induced pans. Two of the studied soils had hard-structured Bt horizons (clay pans), but in only one of them (silty clay loam Abruptic Argiudoll, Villa Lia site) the expected penetrometer resistance increases (up to 9 MPa) were observed with depth. In the other clay pan soil (silty clay loam Vertic Argiudoll, Perez Millan site), penetrometer resistance did not increase with depth but reached 14.5 MPa at 0.075 and 0.2 m depth in CT and ZT plots, respectively. However, maize root abundance was stratified in the first 0.2 m at the Villa Lia and Perez Millan sites. There, the hard Bt horizons did not represent an absolute but a relative mechanical impedance to maize roots, by the observed root clumping through desiccation cracks.

Residue Decomposition and Fate of Nitrogen‐15 in a Wheat Crop under Different Previous Crops and Tillage Systems

Abstract Nitrogen (N) management may be improved by a thorough understanding of the nutrient dynamics during previous‐crop residue decomposition and its impact on fertilizer N fate in the soil–plant system. An experiment was conducted in the Argentine Pampas to evaluate the effect of maize and soybean as previouscrops and plow‐till and no‐till methods on N dynamics and 15N‐labeled fertilizer uptake during a wheat growing season. Maize and soybean residues released N under both tillage treatments, but N release was faster from soybean residues and when residues were buried by tillage. Net immobilization of N on decomposing residues was not detected. A regression model that accounted for 92% of remaining N variability included time, previous crop, and tillage treatment as independent variables. The rapid residue decomposition with N release was attributed to the high temperatures of the agroecosystem. The recovery of 15N‐labeled fertilizer in the wheat crop, soil organic matter, and decomposing residues was not statistically different between previous crop treatments or tillage systems. Crop uptake of fertilizer N averaged 52% across treatments. Forty percent of fertilizer N was removed in grains. Immobilization of labeled N on soil organic matter was substantial, averaging 34% of the 15N‐labeled fertilizer retained, but was very small on decomposing residues, averaging 0.2–3.0%. Fertilizer N not accounted for at harvest in the soil–plant system was 12% and was ascribed to losses. Previous crop or tillage system had no impact on wheat yield, but when soybean was the previous crop, N content of grain and straw+roots increased. Discussion is presented on the potential availability of N retained in wheat straw, roots, and soil organic matter for future crops.

Topsoil structure in no-tilled soils in the Rolling Pampa, Argentina

Some topsoil physical properties evolve unfavourably under continuous, no-till farming. On the Pampa, loam soils under no-till sometimes have lower infiltration rates than those conventionally tilled; this is due to the occurrence of platy and massive structures. In this study, we aimed to identify the soil management practices that promote platy structure formation, and explain the soil physical behaviour linked to the thickness of platy structures in relation to infiltration rate, bulk density and shear strength. Six fields with different numbers of years under agriculture and diverse previous crops (maize or wheat-soybean double crop) were sampled, distinguishing within each field headlands (areas with higher traffic) and centre (lower traffic). Twenty samples were taken at random along a 200-m transect to characterise soil structure (platy, granular or massive) and the thickness of the platy structure. Principal component analysis revealed linkages between previous crop and location in each field and type of structure. ANOVA showed a significant (P <0.05) interaction of previous croplocation. The frequency and thickness of the platy structures were lower, and those of granular structures higher, under wheat-soybean double cropping and in the centre of the field. Greater thickness of the platy structure determined lower water infiltration rate (r=-0.337; P <0.01) and greater soil shear strength (r=0.297, P <0.01). Micromorphological analysis indicated the dominance of massive and platy structure in the headlands and bioturbation in the centre of the fields with wheat-soybean double cropping. These results suggest bioturbation, crop- root binding and low machinery traffic as the main factors minimising soil evolution towards unfavourable structural types under no-till farming in the area.

Soil nitrous oxide emissions under different management practices in the semiarid region of the Argentinian Pampas

The aim of this study was to analyze the influence of different crop sequences (soybean-corn and soybean–soybean) and tillage systems (no tillage and reduced tillage) on nitrous oxide (N2O) soil emissions under field conditions. The experiment was carried out in Manfredi, Córdoba province, Argentina on an Entic Haplustoll and N2O emissions were measured in the field during a year. N2O fluxes were low during winter, but in late spring it peaked. For fallow, N-NO3-content was the most important variable to explain N2O emissions. For growing period water-filled pores was the main variable explaining N2O emissions. Nitrogen fertilization of corn crop increased N2O-N emissions, whereas no significant differences were found due to the tillage system. Measured annual N2O-N emissions were generally lower than those calculated using the methodology proposed by the Intergovernmental Panel on Climate Change.

Cover crops in the agricultural systems of the Argentine Pampas

The Argentine Pampas (figure 1) is located in the south cone of South America (31° to 39° S and 58° to 65° W). The region extends along 55 to 60 million ha (135 to 148 million ac) and was originally covered with temperate grasslands. The region shows several similarities as well as differences with their equivalent grassland of North America. In a simplified picture, the climate is humid in the east and subhumid/semiarid in the west. Rainfall varies from 1,200 mm y−1 (47 in yr−1) in the east to 500 mm y−1 (20 in yr−1) in the west. Fall and spring/summer are the rainier seasons but there is considerable variability in monthly and annual precipitation. The Pampas is classified as mesothermal, with average temperature around 14°C (57.2°F) in the south and 20°C (68°F) in the north. Winters can be cold, especially in the south, where it sometimes snows, but soils never become frozen. Most soils of the Pampas were developed from loess-like sediments and are mainly Mollisols. From east to west, soils are mainly Argiudolls, Hapludolls, and Haplustolls, and in some localized areas Natraquolls. Other less representative soils are Alfisols, Vertisols, and Entisols (Lavado and Taboada 2009). The Pampas is…