Effi cient use of N in plant production is an essential goal in crop management. An experiment was performed at INTA Balcarce, Argentina during 3-yr to evaluate the eff ect of row spacing and N level on nitrogen use effi ciency (NUE) in no-till (NT) irrigated maize (Zea mays L.). Treatments consisted of a factorial combination of row width (70, 52 and 35 cm) and N rates (0 to 180 kg N ha −1 ). Nitrogen rate and narrow rows increased total dry matter (DM), grain yield, and N accumulation. Relative responses to narrow rows decreased as N availability increased. Th e NUE decreased with N rate and increased with narrow row spacing. Narrow rows increased NUE by 12 and 15% expressed as DM or grain yield per unit of available N, respectively. Physiological effi ciency decreased with N rate and was not aff ected by row spacing. Recovery effi ciency (RE) decreased with increasing N rate, and increased for the narrow row spacings. Th e eff ect of narrow rows on RE decreased as N availability increased. Narrow rows increased NUE largely as a result of increased RE. Th ese increments in RE could contribute to increase the profi tability of maize production and to diminish the risk of environmental pollution.

Sustainability is influenced in many production systems by the variation of soil organic C (SOC) content and dynamics, and crop rotations. We hypothesized that arable layer SOC under conventional tillage can be managed through the amount of residue C (RC) returned to the soil as affected by tillage and fertilization. Soil organic C dynamics of a complex of Typic Argiudoll and Petrocalcic Paleudoll soils under conventional tillage between 1984 and 1995 at Balcarce, Argentina was studied for 16 crop sequences. Crops included were spring wheat (Triticum aestivum L.), soybean [Glycine max (L.) Merr.], sunflower (Helianthus annuus L.), and corn (Zea mays L.). Eleven years of conventional tillage decreased SOC 4.1 to 8.8 g kg−1 without supplemental N and 2.8 to 7.2 g kg−1 when N fertilizer was applied. Soil organic C loss increased when soybean (1.2 Mg RC ha−1 yr−1) was present in the sequence and decreased when corn (3.0 Mg RC ha−1 yr−1) was present. The amount of RC returned by the sequences correlated with SOC in 1995 and with SOC at equilibrium , but the sequences with two summer crops (soybean, sunflower, or corn) every 3 yr showed lower SOC in 1995 (28.9–33.8 g kg−1) and at equilibrium (24.0–34.4 g kg−1) than sequences with none or one summer crop (29.7–35.0 g kg−1 either in 1995 or at equilibrium) for the same range of RC (1.4–2.6 Mg RC ha−1 yr−1). The difference between sequences in the relationship between RC and SOC were attributed to tillage timing. Under conventional tillage, arable layer SOC can be managed through the selection of the crops in the rotation and N fertilization, but the timing and intensity of tillage have to be taken into account.

efficiency of light interception per unit leaf area (Bullock et al., 1988; Westgate, 1998). However, grain yield No-tillage maize (Zea mays L.) grown without adequate N interincreases in response to narrower rows under convencepts less than 95% of the incident radiation at flowering. Reducing tional tillage and adequate N were less than 10% the distance between rows could increase radiation interception and grain yield. A 2-yr experiment was conducted at INTA Research (Hunter et al., 1970; Stivers et al., 1971; Bullock et al., Station, Balcarce, Argentina (37؇45 S; 58؇18 W), to study the effect 1988; Porter et al., 1997). of row spacing and N availability on intercepted radiation, kernel In Balcarce, no-tillage maize grown without adequate number, and grain yield of no-till maize. Treatments consisted of a N intercepts less than 95% of incident radiation at flowfactorial combination of row width (0.35 and 0.70 m) and N (0 and ering (Sainz Rozas, 1997). Decreasing the distance be-120 or 140 kg ha Ϫ1 each year) at a constant plant density. Low N tween rows could partially offset N stress effects and decreased kernel number and grain yield. Narrow rows significantly increase kernel number per square meter and grain increased kernel number per unit area and grain yield. Average inyield. Because LAI and radiation interception respond creases in response to narrow rows were 14.5 and 20.5% for kernel to N supply (Novoa and Loomis, 1981; Lemcoff and number and grain yield, respectively. However, relative increases in Loomis, 1986; Muchow, 1988; Muchow and Davis, 1988; dale, 1995) because of an increase in LAI and in the a.i. ha Ϫ1 ; and metolachlor (2-chloro-N-(2-ethyl-6-methylphe-

processes (Touraine et al., 1994), a high N requirement during late winter and early spring has been observed For some species, mathematical models have been developed to for annual ryegrass (Salette et al., 1984). However, since describe tissue N dilution during crop growth and to estimate the the rate of soil N mineralization varies throughout the plant N status applying the N nutrition index (NNI), the ratio betweenyear, mainly in response to soil temperature and water the actual tissue N concentration and the tissue N concentration needed to obtain the maximum instantaneous crop growth rate (criti-

Inclusion of pastures in a rotation may reverse the effects of conventional cropping and tillage on soil degradation. We hypothesized that crop‐pasture rotations could be defined with a minimum pasture component and a maximum conventional cropping component to accomplish sustainable management. Soil organic carbon (SOC), light‐fraction carbon (LFC), microbial biomass nitrogen (MBN), and aggregate stability index (ASI) were measured in the surface 0 to 0.15 m of a fine, mixed, thermic Typic Argiudoll with 2% slope under a long‐term crop rotation experiment at Balcarce, Argentina. Treatments were continuous cropping and crop‐pasture (50:50 and 75:25) rotations. All seedbeds were prepared with conventional tillage. All soil quality indicators decreased with cropping and increased with pasture. Data for each variable were fitted to an exponential model to describe their variation in time. Soil organic C decreased 4.4 g kg−1 in 6 to 7 yr under cropping and rose to the original level (37.2 g kg−1) after 3 to 4 yr under pasture. Light‐fraction C, MBN, and ASI fell 0.9 g kg−1, 39.0 mg kg−1, and 43.9, respectively (97–100% of the estimated decline), after 7 yr under cropping, while they recovered to the values at the beginning of the cropping period (1.8 g kg−1, 99.5 mg kg−1, and 76.7, respectively) in a few years under pasture. Rotations including a maximum of 7 yr of conventional cropping alternated with a minimum of 3 yr of pasture would maintain soil properties within acceptable limits and meet the goals of sustainable agriculture under conditions similar to this experiment.