2.1. Study Area and Experiment
The experiment reported herein involved an irrigated, grain maize (Zea mays L.)-forage oats (Avena sativa L.) rotation and tested several soil tillage practices and residue retention levels. It was located at the “San Luis” Experiment Station of the Instituto Nacional de Investigaciones Forestales, Agricolas y Pecuarias (INIFAP) in Soledad de Graciano Sánchez, San Luis Potosí, Mexico (22°13′35.30″ N 100°50′56.67″ W, 1838 m above sea level).
At the start of the experiment in 1997, the soil was characterized as a xerosol with a sandy clay loam texture and 1.4% organic matter, pH of 8.1 and electric conductivity of 0.81 dS m−1
. The electric conductivity of irrigation water was 0.29 dS m−1
and it had a sodium ratio of 1.26. The region has a dry temperate climate with an annual average temperature of 16.8 °C, a frost-free period from April to September, and an annual average rainfall of 332 mm (Figure 1
). Monthly averages of precipitation and temperature from 1997 to 2018 were taken from the weather station closest to the trial (Supplementary Table S1
The seven treatments evaluated consisted of four tillage methods: three conventional and one comprising four variants of permanent raised beds (PB) (Table 1
). The three conventional methods were (1) inversion tillage with a disc plow and disk harrowing to a depth of 30 cm (P+D), (2) disk harrowing to a depth of 30 cm (D), and (3) non-inversion tillage with a “multiplow”, a ripper equipped with sweeps to break up the soil at the operating depth (Supplementary Figures S1 and S2
), followed by disk harrowing to a depth of 30 cm (M+D). In all three conventional tillage treatments, 1.65 m wide beds were shaped after disking; because all three treatments were disked, the soil was left in a similar condition, with clods of up to 5 cm on the surface. In treatments P+D, D, and M+D, soil preparation took place at the beginning of each maize and oats cycle. The PB were created in 1996 and were 1.65 m wide. Furrows in the PB have been reshaped every five years; the tops of the beds were not tilled during the study period. Furrows in all treatments were approximately 10 cm deep, with some variation in the periods between reshaping (every five years in PB) or remaking (every season in tilled treatments) of the furrows, when they gradually become shallower.
Four levels of soil cover with maize residues were tested on the PB: zero (PB0%), one-third (PB33%), two-thirds (PB66%) and full soil cover (PB100%). The amounts of residue to cover one-third, two-thirds and the entire soil surface correspond to 1.3, 2.6, and 4.0 Mg ha−1, respectively. Residues were applied once a year after maize harvest. For oats, only a 5–10 cm tall stubble was retained. All maize residues were removed after harvest, chopped to 15–20 cm long pieces, and appropriate amounts were returned to the soil. No residue was returned to treatments P+D, D and M+D.
Plots were 8.25 m wide and 30.0 m long. Treatments were evaluated in a randomized complete block design with two replications.
Maize was planted in the spring and oats in the fall each year. In the summer cycles of 2009, 2010 and 2011, and in the winter cycles of 2007-2008, 2008–2009, 2009–2010, 2010-2011 and 2011–2012, the trial lay fallow with no activities, due to budget constraints. In the 2002-2003 oats cycle the crop was lost because of frost damage. Different maize hybrids were used in two periods of the trial, each at their recommended planting density. From 1997 to 2008, white maize hybrid H-311 (INIFAP, Mexico) was used, with a planting density of 50,000 plants ha−1. From 2011 to 2017, the white maize hybrid XR-45 (Ceres, Mexico) was planted at 69,000 plants ha−1. Maize row spacing was 0.825 m. The oat cultivar used was “Chihuahua” with a seeding rate of 80 kg ha−1. Oats were planted in two double rows per bed. The center of the double rows was 0.825 m apart, with 0.20 m between each single row within a double row. Planting was done with a zero tillage planter. The fertilization dose for maize and oats was 200-100-00 (kg ha−1 N-P-K) and 80-40-00 respectively, applying half of N and all of P at sowing time and the other half of N at approximately 35 days after planting.
Before sowing maize and oats, weeds in the PB treatments were controlled with glyphosate (1.4 L a.i. ha−1), whereas in the tilled treatments, weeds were controlled by the tillage operations. In the maize crop, atrazine (0.75 L a.i. ha−1) was applied in the first two weeks after planting in all treatments. Additionally, weeds were controlled mechanically in the tilled treatments approximately 35 days after planting, whereas in the PB treatments manual control was used if necessary. In 2017, nicosulfuron (60 g a.i. ha−1) was applied to control grass weeds in the PB treatments. In the oats season, weed pressure was generally low and only mechanical or manual control was used when necessary in all treatments.
The crops were irrigated according to the conventional local practice, approximately every 18 days by furrow irrigation. Maize was irrigated every 14 days during grain filling. When it rained, irrigation was postponed according to the recorded rainfall. All treatments were irrigated simultaneously. In general, six irrigations of an estimated quantity of 1,000,000 L ha−1 (100 mm) of water were applied per cropping cycle for both crops. Water was applied to the furrows but in treatments with low infiltration rates could cover the top of the beds as well.
Maize and oat forage yields were determined in sampling areas of 10.2 m2. Maize was harvested at 14% grain moisture content and oats were harvested when the grain reached milk or dough stage. Oats moisture content was approximately 70% at harvest, yields were reported as dry biomass. Oats outside the sampling area were harvested with a cutter bar, leaving 5 to 10 cm of stubble on the field. Plant height in maize was measured from the ground to the base of the flag leaf.
2.2. Soil Quality
Samples for chemical soil analysis were taken on 24 October 2017 at depths of 0–5 and 5–30 cm. Five samples were taken per plot and a composite sample was made. Soil analyses were carried out by Fertilab, Celaya, Mexico. Phosphorous content was determined by the Bray and Kurt method as described in [17
] and by the Olsen method as described in [18
]. Interchangeable bases (Ca, Mg, Na and K) were determined with the ammonium acetate method, micronutrients (B, Cu, Mn, Fe, Zn) were determined with the DTPA-Sorbitol method at pH 7, and organic matter content was determined with the Walkley and Black method [19
]. Sulphur was determined with the turbidimetric method [17
] and nitrates were determined with the colorimetric method as described in [20
Physical soil quality data and samples for mean weight diameter (MWD) analyses were resampled on 10 October 2018, in the sowing line. A composite sample of five samples per plot from 0–5 cm was analyzed for MWD by dry and wet sieving as described in [21
]. Sand correction was applied. Bulk density was measured by sampling at 0–5 and 15–20 cm and determining volume and oven dry weight for three samples per plot. Penetration resistance was measured with a dynamic penetrometer at depths of 0–15, 15–30, 30 to 45 and 45–60 cm in 4 points per plot. Infiltration was measured as direct surface infiltration by the “time to pond” method as described in [23
]. The time for water to infiltrate during an irrigation was measured for three irrigation events during the 2018 maize crop cycle and was defined as the time between opening the irrigation tube and the moment when no standing water was visible on the surface.