No abstract

Wagner et al. (1998) showed that inorganic C determined as the difference between total C by combustion Soil organic C (SOC) analyses using high temperature induction furnace and SOC where soils are acidified before comfurnace combustion methods have become increasing popular because of advances in instrumentation. Combustion methods, however, also bustion had a close correlation with their volumetric IC include C from CaCO 3 and CaMg(CO 3) 2 found in calcareous soils. system. However, at concentrations Ͻ5.0 g IC kg Ϫ1 , the Separate analysis of the inorganic C (IC) must be done to correct C combustion method difference was less precise. This dedata from combustion methods. Our objective was to develop a effitermination also requires the sample be run twice through cient and precise IC method by modification of the pressure-calcimea dry combustion system which dramatically increases the ter method. We modified the method by using Wheaton serum bottles cost of the analysis. Acidification of soils prior to analysis (20-mL and 100-mL) sealed with butyl rubber stoppers and aluminum by dry combustion is also time-consuming and presents tear-off seals as the reaction vessel and a pressure transducer monidifficulties because destruction of organic matter can tored by a digital voltmeter. Our gravimetric IC determination of six occur (Nelson and Sommers, 1996). soils showed a strong correlation when regressed against IC from the We propose a modification of the reaction vessel which modified pressure-calcimeter method (slope of 0.99, r 2 ϭ 0.998). The method detection limit (MDL) was 0.17 g IC kg Ϫ1 for the 20-mL facilitates effective analysis of batch runs of 80 to 120 serum bottles and the limit of quantification (LOQ) was 0.30 g IC samples per day. Our main objective was to develop a kg Ϫ1. The 100-mL serum bottle had a MDL of 0.42 with a LOQ of fast and routine method that could produce quantitative 2.4 g IC kg Ϫ1. When using a 100-mL Wheaton serum bottle as the analysis of total IC for both extremes of the analytical reaction vessel with a 0.50-g sample size, soils containing up to 120 g range. Five subobjectives were (i) to compare gravimet-IC kg Ϫ1 , which represent a 100 % CaCO 3 equivalent, can be analyzed rically-determined IC with IC determined by our modiwithin the V output range of the pressure transducer. Soil organic C fied pressure-calcimeter method; (ii) to determine the determined by subtraction of IC from total C from combustion analy-MDL and the LOQ; (iii) to evaluate the effect of reducsis correlated well with SOC determined by the Walkley-Black. ing the sample size on precision; (iv) to evaluate the time required to neutralize a range of carbonate-containing

Soil organic C (SOC) has decreased under cultivated wheat (Triticum aestivum)‐fallow (WF) in the central Great Plains. We evaluated the effect of no‐till systems of WF, wheat–corn (Zea Mays)‐fallow (WCF), wheat–corn–millet (Panicum miliaceum)‐fallow, continuous cropping (CC) without monoculture, and perennial grass (G) on SOC and total N (TN) levels after 12 yr at three eastern Colorado locations. Locations have long‐term precipitation averages of 420 mm but increase in potential evapotranspiration (PET) going from north to south. Within each PET location, cropping systems were imposed across a topographic sequence of summit, sideslope, and toeslope. Cropping intensity, slope position, and PET gradient (location) independently impacted SOC and TN to a 5‐cm soil depth. Continuous cropping had 35 and 17% more SOC and TN, respectively, than the WF system. Cropping intensity still impacted SOC and TN when summed to 10 cm with CC > than WF. Soil organic C and TN increased 20% in the CC system compared with WF in the 0‐ to 10‐cm depth. The greatest impact was found in the 0‐ to 2.5‐cm layer, and decreased with depth. Soil organic C and TN levels at the high PET site were 50% less than at the low and medium PET sites, and toeslope soils were 30% greater than summit and sideslopes. Annualized stover biomass explained 80% of the variation in SOC and TN in the 0‐ to 10‐cm soil profile. Cropping systems that eliminate summer fallowing are maximizing the amount of SOC and TN sequestered.

When appraising the impact of food and fiber production systems on the composition of the Earth's atmosphere and the 'greenhouse' effect, the entire suite of biogenic greenhouse gases -carbon dioxide (CO 2 ), methane (CH 4 ), and nitrous oxide (N 2 O) -needs to be considered. Storage of atmospheric CO 2 into stable organic carbon pools in the soil can sequester CO 2 while common crop production practices can produce CO 2 , generate N 2 O, and decrease the soil sink for atmospheric CH 4 . The overall balance between the net exchange of these gases constitutes the net global warming potential (GWP) of a crop production system. Trace gas flux and soil organic carbon (SOC) storage data from long-term studies, a rainfed site in Michigan that contrasts conventional tillage (CT) and no-till (NT) cropping, a rainfed site in northeastern Colorado that compares cropping systems in NT, and an irrigated site in Colorado that compares tillage and crop rotations, are used to estimate net GWP from crop production systems. Nitrous oxide emissions comprised 40-44% of the GWP from both rain-fed sites and contributed 16-33% of GWP in the irrigated system. The energy used for irrigation was the dominant GWP source in the irrigated system. Whether a system is a sink or source of CO 2 , i.e. net GWP, was controlled by the rate of SOC storage in all sites. SOC accumulation in the surface 7.5 cm of both rainfed continuous cropping systems was approximately 1100 kg CO 2 equivalents ha À1 y À1 . Carbon accrual rates were about three times higher in the irrigated system. The rainfed systems had been in NT for >10 years while the irrigated system had been converted to NT 3 years before the start of this study. It remains to be seen if the C accrual rates decline with time in the irrigated system or if N 2 O emission rates decline or increase with time after conversion to NT.

crops every 3 yr) and even continuous (annual) cropping in some instances. For example, annualized grain yields Water is the principle limiting factor in dryland cropping systems. for WCF are 75 to 100% greater than WF (Peterson et Surface soil physical properties influence infiltration and cropping al., 2000). Cropping intensification has been possible systems under no-till management may affect these properties through because no-till practices improve soil water storage effiresidue addition. The objectives of this study were: (i) to determine how cropping intensity and topographic position affect soil bulk den-ciencies in the early phases of fallow (Farahani et al., sity, porosity, sorptivity, and aggregate stability in the surface 2.5 cm 1998). Since nearly 75% of the annual precipitation in of soils at three eastern Colorado sites; and (ii) to relate these properthis region occurs during April to September, relatively ties to crop residue returned to the soil surface. No-till cropping small net increases in soil water storage can provide the systems had been in place on three slope positions, at three sites, for necessary water to sustain crop growth between rainfall 12 yr prior to this study. Wheat (Triticum aestivum L.)-corn (Zea events. Thus water capture via increased water infiltramays L.)-fallow (WCF) and continuous cropping (CC) systems were tion rates becomes a significant factor in maximizing wacompared with wheat-fallow (WF) on summit and toeslope positions ter storage at all points in the system. An added benefit at two sites (Sterling and Stratton), and at the third site (Walsh) of cropping intensification is that increased amounts of wheat-sorghum [Sorghum bicolor (L.) Moench]-fallow (WSF) recrop residue are returned to the soil capared with WF. placed WCF. Cropping systems (CC and WCF or WSF) that returned We believe this residue may greatly improve soil physimore crop residue decreased bulk density and increased total and cal properties resulting in increased water infiltration effective porosities compared with WF. Site and slope positions that and capture efficiency. produced more crop residue also improved these properties. However, Soil physical properties such as bulk density, porosity, sorptivity developed no significant differences as a result of cropping system. Macroaggregates made up a higher percentage of total aggre-sorptivity, and aggregation dictate the infiltration chargates in CC and WCF or WSF compared with WF in proportion to acteristics and potentials of the soil. Most important are residue added and were also a function of clay content of the soil at the physical properties of the surface soil (top 2.5 cm), different sites and slope positions. These factors enhance the potential as this is the initial soil-water interface. However, longfor greater infiltration and hence greater water availability for crops. term infiltration can be affected by the hydraulic conductivity characteristics of deeper soil layers. Site latitude (evaporation potential), landscape slope, and cropping

the slow pool, with decadal turnover times, while Ͻ5% of the SOC is found in the rapidly cycling active fraction, Previous studies of no-till management in the Great Plains have with turnover times ranging from hours to months (Folshown that increased cropping intensity increased soil organic carbon (SOC). The objectives of this study were to (i) determine which soil

a function of C inputs and losses (Campbell et al., 2000a(Campbell et al., , 2000b. Inputs, primarily as photosynthates, are added Summer fallow (fallow) is still widely used on the North American either directly (e.g., crop residue) or indirectly (e.g., Great Plains to replenish soil moisture between crops. Our objective was to examine how fallowing affects soil organic carbon (SOC) in animal manure derived from plant C). Losses occur various agronomic and climate settings by reviewing long-term studies mostly as CO 2 from decomposition. Summer fallow rein the midwestern USA (five sites) and the Canadian prairies (17 duces C storage in several ways (Janzen et al., 1998). sites). In most soils, SOC increased with cropping frequency though First, frequent summer fallow usually reduces inputs of not usually in a linear fashion. In the Canadian studies, SOC response photosynthetically derived C into soils because there to tillage and cropping frequency varied with climate-in semiarid are no plant C inputs (except via weeds) during the conditions, SOC gains under no-till were about 250 kg ha Ϫ1 yr Ϫ1 fallow phase. Second, it may enhance the rate of minergreater than for tilled systems regardless of cropping frequency; in alization of soil organic matter to CO 2 because it keeps subhumid environments, the advantage was about 50 kg ha Ϫ1 yr Ϫ1 for the soil wetter (and perhaps warmer) for longer periods. rotations with fallow but 250 kg ha Ϫ1 yr Ϫ1 with continuous cropping. Further, if tillage is used to control weeds during sum-Specific crops also influenced SOC: Replacing wheat (Triticum aestivum L.) with lentil (Lens culinaris Medikus) had little effect; replacing mer fallow, decomposition may be accelerated by diswheat with lower-yielding flax (Linum usitatismum L.) reduced SOC ruption of soil aggregates and exposure of organic matgains; and replacing wheat with erosion-preventing fall rye (Secale ter to microbial activity. Summer fallow may also cereale L.) increased SOC gains. In unfertilized systems, cropping accelerate soil C losses by erosion, but these losses are frequency did not affect SOC gains, but in fertilized systems, SOC localized, often resulting in redistribution of C more gains often increased with cropping frequency. In a Colorado study than in its release to the atmosphere (Gregorich et (three sites each with three slope positions), SOC gains increased al., 1998). with cropping frequency, but the response tended to be highest at Our objective was to review data from long-term exthe lowest potential evaporation site (where residue C inputs were periments in the U.S. and Canadian Great Plains to greatest) and least in the toeslope positions (despite their high residue determine the magnitude of the effect of summer fallow C inputs). The Century and the Campbell et al. SOC models satisfactorily simulated the relative responses of SOC although they underesti-on changes in SOC and to assess how these changes are mated gains by about one-third.

Soil and water conservation is essential to the sustainability of Great Plains dryland agriculture. We hypothesized that cropping intensification improves the efficient use of precipitation. We evaluated long‐term observations of soil water at three locations in eastern Colorado for a range of pan evaporations (1050–1900 mm), soils, and cropping systems. Soils at various locations were mostly of the Argiustoll subgroup except for one Ustochrept and one Haplargid, both at the higher evaporation location. Normal precipitation at the three locations ranges from 400 to 425 mm yr‐1. Systems included a 2‐yr winter wheat (Triticum aestivum L.)‐fallow (WF) and more intense 3‐yr winter wheat‐corn (Zea mays L.)‐fallow and winter wheat‐sorghum [Sorghum bicolor (L.) Moench]‐fallow and 4‐yr rotations. To quantify the effectiveness of the intensified systems at utilizing precipitation, we introduce the System‐Precipitation‐Storage Index (SPSI) and System‐Precipitation‐Use Index (SPUI). Mean SPSI values were 0.19 and 0.28 for 2‐ and 3‐yr systems, respectively, meaning that the fallow periods in the 3‐yr rotation were collectively 47% more efficient at storing precipitation than fallow in WF. Inclusion of a summer crop, such as corn or sorghum, increased the fraction of precipitation allocated to growing‐season crop production (i.e., SPUI) from 0.43 in WF to 0.56 (i.e., an increase of 30%) in 3‐yr systems. The gains in efficient use of precipitation with intensification resulted from (i) reducing the frequency of the inefficient fallow preceding wheat, and (ii) using water for transpiration that would otherwise be lost during fallow through soil evaporation, runoff, and deep percolation.