Three independent studies were conducted in adjacent experimental (~0.5 ha) fields with <2% slope and contrasting primary tillage [tilled with a chisel plow (Chisel), no-till since 1995 (NT1995), and no-till since 2005 (NT2005)]. These fields were located at the Swan Lake Research Farm (45°41′ N lat; 95°48′ W long; elevation 370 m). The local thirty-year (1971–2000) average annual precipitation is 645 mm and mean monthly temperatures ranges from −13.1 °C in January to 21.7 °C in July [23
]. Each study field had 24 (6.1 m by 22.9 m) plots arranged in a randomized complete block design with three stover return treatments (Full Return, Moderate Return and Low Return), both phases of a corn soybean rotation and four replications. Randomization occurred within a study but not among study fields, so comparison were made within but not among study fields.
2.1. Characterization and History of Study Fields
The field managed with annual chisel plowing (Chisel), had two similarly textured Barnes-Aastad complexes. Using USDA-SCS soil maps [24
] plots were arranged to block soil variability within replications, therefore three replications were on Barnes soil (fine-loamy, mixed, superactive, frigid Calcic Hapludoll) with the fourth replication on Aastad (fine-loamy, mixed, superactive, frigid Pachic Hapludoll). Both the NT2005 and NT1995 fields were established on an area mapped nearly exclusively as Barnes. All fields were managed with a corn/soybean rotation, with both crops present each year.
Prior to 2005, the Chisel field had been moldboard plowed (>20 cm) in the fall after harvest with one or two disking (~10–15 cm depth) operations prior to planting. Beginning in 2005, the moldboard plow was replaced with a chisel plow (~20 cm) but seedbed preparation remained the same. For at least ten years prior to establishing this stover study, the NT1995 had been managed without tillage. During the previous years, both NT1995 and Chisel were planted to continuous corn (10 years) or in a corn-soybean rotation (four years). The pre-experimental cropping history for NT2005 was a corn, soybean, wheat (Triticum aestivum L.) rotation with moldboard plow tillage (>20 cm) each fall after harvest for at least 10 years. This field was last moldboard plowed in 2004.
2.3. Agronomic Management
All fields were planted with glyphosate-tolerant corn (Croplan 296TS) and soybean (508-M8) with both crop phases present each year. Planting density was 78,000 (Chisel) and 81,500 plants ha−1
(NT19995 and NT2005) for corn and 247,400 plants ha−1
for soybeans in all fields. Listing even-year crop first, the rotations were designated corn-soybean or soybean-corn. Corn plots received 10 and 15 kg N and P of starter fertilizer in 2005 through 2008. Prior to planting in 2009 to 2011 all plots in all fields received knife-injected (5–7 cm) 21, 34, 65 and 52 kg ha−1
of N, P, K and S, respectively; plus, an additional 4 and 6 kg ha−1
of N and P was applied when planting corn. Nitrogen fertilizer was applied annually during the corn phase as side-dressed anhydrous ammonia with average annual application rates of 136, 139 and 148 kg N ha−1
applied in Chisel, NT2005 and NT1995, respectively. Side-dress rates were based on spring soil tests (0–30 cm) and the ARS Nitrogen-decision aid [25
]. Weeds were controlled in both crops with two glyphosate applications annually; insecticides were only applied to soybean.
Corn and soybean grain yield at standard grain water contents of 15.5 and 13 g kg−1
, respectively were based on harvest with a two-row plot scale combine. Soybean straw production was determined on plants collected from a 0.76 m2
area prior to leaf-drop R6 [26
]. At physiological maturity, corn was collected from 1.5 m2
to determine stover yield. Corn stover and soybean straw were reported as dry mass per area, based on oven-dried (60 °C) mass. Harvest index [27
] was calculated as dry grain divided by dry grain plus stover.
2.4. Soil Parameters
In the fall of 2005, baseline soil samples were collected for all fields using a Giddings (Giddings Machine Company, Windsor, CO, USA) hydraulic probe (5.33 cm i.d.) to 100 cm as recommended by Liebig et al.
]. Three soil cores taken in each plot were divided into six depth increments (0–5, 5–10, 10–20, 20–30, 30–60, and 60–100 cm). Soil from the 0–5 and 5–10 cm depth increments was passed through a 2 mm sieve. A subsample (~50-g) was air-dried for POM isolation while soil was oven-dried at 37 °C for chemical analyses. Soil from one core was used for determining soil bulk density at depth intervals below 10 cm. Cores were assumed to be uncompressed when the core length equaled the whole depth [29
]. In the surface 0–5 and 5–10 cm increments, a hand-held soil probe (5 cm i.d.) was used to collect a sample for bulk density. Soil texture was determined using the hydrometer method [30
]. Soil pH (1:1 CaCl2
], total C and N (LECO TRU-SPEC CN analyzer; LECO Corporation, St. Joseph, MI), inorganic C [33
], Olsen P and extractable K [34
] were determined. Organic C was calculated as the difference between total combustible C and inorganic C as these are calcareous soils. Particulate organic matter was isolated from air-dried soil as described by Cambardella and Elliot [16
]. The mass of POM independent of sand [35
] was determined by weight loss on ignition using a method by Schulte [36
] as reported by Cambardella et al.
]. In 2009, to obtain an early prediction of changes in soil organic matter due stover return rate, POM was isolated and bulk density determined from soil collected at the surface 0–5 and 5–10 cm as described above. As previously described for baseline samples, in the fall of 2010 (following harvest and preceding tillage in Chisel), soil samples were collected to 100 cm depth and assessed for bulk density, pH, total C and N by combustion, inorganic C, Olsen P and extractable K.
Percent soil coverage and dry aggregate size distribution were used as indicators for treatment induced changes in soil erosion potential. Soil coverage was assessed annually by the transect method [37
]. The transect method, which does not require a minimum residue size, counts the number of times a 15 m tape with 100 equidistant marks intersects visible residue. Soil for dry aggregate size distribution was collected in the summer of 2011 within about 5 cm of the soil surface after removing visible surface residue. Dry aggregate size distribution using the method described by Chepil [39
] and Pikul et al.
] was determined using a rotary sieve operating at 6-rpm to separate air-dried soil into six aggregate size groupings: 0–0.5, 0.5–1.0, 1–2, 2–3, 3–5, 5–9, and 9–20 mm. Total soil mass in aggregates <20 mm and mass of each aggregate size was determined.
Biological parameters (microbial biomass and composition and enzyme activities) were evaluated on soil samples collected in 2008 prior to spring agronomic operations from Full and Low Return in all fields using a hand-held probe. Soil cores were divided into 0–5 and 5–10 cm depth increments. Soil MBC and MBN were determined on the field-moist samples, with a dry equivalent mass of 15-g, by the chloroform-fumigation-extraction method using 0.5 M K2
as an extractant [41
]. Organic C and N were quantified using a CN analyzer (Shimadzu Model TOC-V/CPH-TN; Shimadzu Scientific Instruments, Columbia, MD, USA). Microbial biomass C was calculated as (organic C extracted from fumigated soil minus organic C extracted from non-fumigated soil) divided by a kEC
factor of 0.45 [43
]. The soil MBN was calculated similarly, but used a kEN
factor of 0.54 [44
]. These factors correct for biomass solubilized during the fumigation-extraction [45
]. Each sample had duplicate analyses and results are expressed on a dry-weight basis. Soil water content was determined after drying soil at 105 °C for 48 h.
Soil microbial community structure was characterized using fatty acid methyl esters (FAME) analysis on field-moist soil samples using the Microbial Identification System (MIS, Microbial ID, Inc., Newark, DE, USA) procedure as applied for soil analyses [46
]. Briefly, the method consists of four steps: (1) saponification of fatty acids in 3 g field-moist soil with 3 mL 3.75 M NaOH (methanol:water, 1:1) solution under heat (100 °C) for 30 min; (2) methylation of fatty acids by adding 6 mL of 6 M HCl in aqueous methanol (1:0.85) under heat (80 °C) for 10 min; (3) extraction of the FAME with 3 mL of 1:1 hexane:methyl-tert butyl-ether solution and rotating the samples end-over-end for 10 min; and (4) washing the organic phases with 1.2 % diluted NaOH by rotating the tubes end-over-end for 5 min. The organic phase was analyzed in a 6890 GC Series II (Hewlett Packard, Wilmington, DE) equipped with a flame ionization detector and 25m × 0.2 mm fused silica capillary column using ultra high purity hydrogen as the carrier gas. The temperature program was ramped from 170 °C to 250 °C at 5 °C min−1
. Fatty acids were identified and their relative peak areas (percent) were determined with respect to the other fatty acids in a sample using the MIS Aerobe method of the MIDI system (Microbial ID, Inc., Newark, DE, USA).
Enzyme activities, β-glucosidase, β-glucosaminidase, and acid phosphatase were assayed as indicators of C, C and N, and P biogeochemical cycling potential, respectively. These enzyme activities were assayed using 1g of air-dried soil with their appropriate substrate and incubated for 1 h (37 °C) at their optimal pH as described by Tabatabai [48
] and Parham and Deng [49
]. Enzyme activities were assayed in duplicate with one control, to which substrate was added after incubation and subtracted from the sample value.