Stephen D. Sparrow

Impact of forest conversion to agriculture on carbon and nitrogen mineralization in subarctic Alaska

Land-use change is likely to be a major component of global change at high latitudes, potentially causing significant alterations in soil C and N cycling. We addressed the biogeochemical impacts of land-use change in fully replicated black spruce forests and agricultural fields of different ages (following deforestation) and under different management regimes in interior Alaska. Change from forests to cultivated fields increased summer temperatures in surface soil layers by 4–5 °C, and lengthened the season of biological activity by two to three weeks. Decomposition of a common substrate (oat stubble) was enhanced by 25% in fields compared to forests after litter bags were buried for one year. In-situ net N mineralization rates in site-specific soil were similar in forests and fields during summer, but during winter, forests were the only sites where net N immobilization occurred. Field age and management had a significant impact on C and N mineralization. Rates of annual decomposition, soil respiration and summer net N mineralization tended to be lower in young than in old fields and higher in fallow than in planted young fields. To identify the major environmental factors controlling C and N mineralization, soil temperature, moisture and N availability were studied. Decomposition and net N mineralization seemed to be mainly driven by availability of inorganic N. Soil temperature played a role only when comparing forests and fields, but not in field-to-field differences. Results from soil respiration measurements in fields confirmed low sensitivity of heterotrophic respiration, and thus decomposition to temperature. In addition, both soil respiration and net N mineralization were limited by low soil water contents. Our study showed that (1) C and N mineralization are enhanced by forest clearing in subarctic soils, and (2) N availability is more important than soil temperature in controling C and N mineralization following forest clearing. Projecting the biogeochemical impacts of land-use change at high latitudes requires an improved understanding of its interactions with other factors of global change, such as changing climate and N deposition.

Decomposition in forest and fallow subarctic soils

SummaryLarge-scale argicultural development in high latitude regions could lead to large losses of soil C due to accelerated decomposition. Changes in decomposition rates of forest floor material upon land clearing in interior Alaska were simulated by measuring, over a 2-year period, changes in mass, cellulose, lignin, and N of forest floor materials and in mass of filter papers and wood in a forest floor and a fallowed field. All materials decomposed slowly at the surface, with about 90% of the original weight remaining after 2 years. Decomposition rates were higher for materials buried in the field than the forest. Cellulose loss in forest floor materials closely followed mass loss, whereas lignin loss was not significant. However, weight loss of wood was rapid when buried in the field, with about 20% of the initial mass remaining after 2 years. Relationships between mass loss of buried forest floor materials and soil degree days were significant (r=70%–80%). Temperature was a major, but not the only factor, controlling decomposition rates. Forest floor materials showed significant N losses, indicating net N mineralization and that N deficiency was not a factor affecting decomposition. C loss to the atmosphere due to decomposition of forest floor materials after forest clearing will be minimal and similar to that in the undisturbed forest if left on the soil surface, but will be substantial if incorportated into the soil. Incorporation is necessary for cropping; thus some accelerated decomposition is unavoidable in clearing subarctic forests for cultivation.

Carbon and nitrogen mineralization in subarctic agricultural and forest soils

SummaryC and N mineralization potentials were determined, in a 12-week laboratory incubation study, on soil samples obtained from recently cleared land which had been cropped to barley for 4 years (field soils) and from nearby undisturbed taiga (forest soils). Treatments for the cropped soils were conventional and no-tillage with and without crop residues removed. An average of about 3% of the total C was evolved as CO2 from the field soils compared with > 10% and 4% for the upper (Oie) and lower (Oa) forest-floor horizons, respectively. Significantly more C was mineralized from the Ap of the no-till treatment with residue left on the surface than from the other field Ap horizons. Both forest-floor horizons showed rather long lag periods for net mineralization compared with the field soils, but at the end of the incubation, more mineral N was recovered from the forest Oie despite a rather wide C:N ratio, than from the field soils. After 12 weeks about 115, 200 and 20 μg mineral N/g soil were recovered from the field Ap, the forest Oie and the forest Oa horizons, respectively. Very little C or N was mineralized from the B horizon of the forest or the field soils. Nitrification was rapid and virtually complete for the field soils but was negligible for both forest-floor O horizons.

Effects of migratory geese on nitrogen availability and primary productivity in subarctic barley fields

Abstract Migratory geese affect agricultural production by removing biomass and by depositing fecal nutrients. This study used 15N as a tracer to examine the quantitative effects of goose fecal N contributions on agricultural production. Barley (Hordeum vulgare cv. Datal) was grown for the production of 15N-labeled grain and straw. Two Canada geese (Branta canadensis Taverners) were fed the grain after harvest to obtain 15N-labeled and unlabeled feces. Net N mineralization and micro-plot studies both indicated that in comparison to barley grain and straw, goose feces provided the greatest amount of available N to the soil and to the subsequent crop, and consequently higher barley yields (59% and 62% increase, respectively). However, C mineralization was greater from grain, with 56% evolved compared to 49% and 26% for feces and straw, respectively. Goose feces also provided the greatest addition of N for the barley plants, with fertilizer N recovery efficiency (FNRE) of 16%, compared to FNRE of 10% from the grain amendment, and 1.2% from the straw amendment. The amount of N available in fecal material from leftover grain consumed by grazing geese is small in comparison to total crop needs, but is a potential source of mineral N during the critical early growth of crops grown in cold, high-latitude soils.

Phosphorus and nitrogen dynamics during field incubations in forest and fallow subarctic soils

A knowledge of the nutrient dynamics that occur with land use changes, e.g., in clearing forests for farmland, is useful in choosing the most efficient soil and fertilizer management practices. To determine net in situ P and N mineralization and nitrification rates of forest floor materials and their nutrient value for agricultural crops, plastic bags containing different materials (moss, O horizon, and A horizon) collected from a subarctic black spruce (Picea mariana Mill.) forest were incubated for 2 years in their respective forest horizons and at 7.5 cm depth in a nearby fallow field. Net amounts of P and N mineralized were highest in moss and were similar in forest and field when the temperature and moisture content were similar, but smaller in forest when the water content was higher. Net nitrification was negligible in O and A horizon material but significant in moss during the 2nd year, occurring sooner and producing higher NOinf3sup-levels in the field (171 mg ha-1) than in the forest (13 mg ha-1). Moss P and N mineralization rates were correlated in the fallow field. Temperature, moisture content, and substrate quality were important factors controlling P and N dynamics of forest floor materials in a subarctic fallow field and native forest. In subarctic regions, incorporation and mineralization of forest floor materials could provide an early source of N and P (70 and 17 kg ha-1, respectively) for succeeding agricultural crops.

Comparison of Methods for Extracting Adenosine Triphosphate from Three High-Latitude Soils

Adenosine triphosphate (ATP) in soil has been used as an index of soil microbial biomass, but efficient extraction of ATP from soil is often a problem. The objective of this study was to find an AT...

Phosphorus mineralization in subarctic agricultural and forest soils

SummaryPotential P and C mineralization rates were determined in a 12-week laboratory incubation study on subarctic forest and agricultural soil samples with and without N fertilizer added. There was no significant difference in net inorganic P produced between N fertilized and unfertilized soils. The forest soil surface horizons had the highest net inorganic P mineralized, 32 mg P kg-1 soil for the Oie and 17 mg P kg-1 soil for the Oa. In the cropped soils net inorganic P immobilization started after 4 weeks and lasted through 12 weeks of incubation. Cumulative CO2−C evolution rates differed significantly among soils, and between fertilizer treatments, with the N-fertilized soils evolving lower rates of CO2−C than the unfertilized soils. Soils from the surface horizons in the forest evolved the highest rates of CO2−C (127.6 and 89.4 mg g-1 soil for the Oie and Oa horizons, respectively) followed by the cleared uncropped soil (42.8 mg g-1 soil C), and the cropped soils (25.4 and 29.0 mg g-1 soil C). In vitro soil respiration rates, or potential soil organic matter decomposition rates, decreased with increasing time after clearing and in accord with the degree of disturbance. Only soils with high potential C mineralization rates and high organic P to total P ratios, mineralized P by the end of the study. Mineralizable P appeared to be associated with readily mineralizable organic C.

Soil quality under different land uses in a subarctic environment in Alaska

As the demand for food, feed, fiber, and, most recently, bio-energy increases, more land may be converted from native conditions for arable uses. Our objective was to evaluate soil quality under native forest, arable land being used for continuous barley (Hordeum valgare L.) production and land that had been tilled and subsequently enrolled in the Conservation Reserve Program (CRP) for at least 18 years. Several physical, chemical, and biological soil quality indicators were measured and a soil deterioration index (DI) was calculated using forest soil as the reference. Results indicated that most organic matter under forest resided on soil surface and was not mixed with mineral soil due to lack of activities by large soil fauna (e.g., earthworms). Soil samples from disturbed areas had a higher organic matter content, which caused most soil quality indicators to be considered ‘improved’ and resulted in better DIs for agricultural and CRP land than for forest soils. This study emphasized the importance of choosing an appropriate reference point for soil quality assessments, especially when data representing one or more key soil processes are missing.

Soil properties and barley yield under a twenty-years experiment of tillage, straw management and nitrogen application rate in the sub-arctic area of Alaska

Abstract A tillage and straw management study was started near Delta Junction Alaska (64°49′N, 147°52′W) USA in 1983 to determine the impact of tillage, straw management, and nitrogen fertilizer application rate on soil properties and barley (Hordeum vulgare L.) grain yield. In October 2003, soil samples were collected from the 0–5, 5–10, 10–25 cm depths in the treatments: no tillage (NT); disked once each spring (DO); disked twice (each for spring and fall, respectively) (DT); straw and stubble retention (SS); straw and stubble removal (NSS); and 11 and 131 kg N ha−1 nitrogen fertilizer application rates, to determine soil total nitrogen and carbon concentration, cation exchange capacity, mineral N, Mehlich-3 extractable phosphorus, exchangeable potassium, pH, electrical conductivity and bulk density. From 1983 to 2003 grain yield was measured from each treatment except in those years in which yield was lost due to birds, weeds, or chemical fallow. The no-tillage treatment tended to increase soil total organic C and N concentrations at 0–5 cm depths. Soil bulk density (0–5 cm) was lower with NT (p<0.05) than with DT. Retaining straw (SS) on the soil surface increased soil organic C concentration at 5–10 cm depth (p=0.06). Mineral N concentration in soil was higher with NT at 0–5 cm depth (p=0.05). Barley grain yield of NT was better than that of DT but varied with time. Rate of N application increased grain yield up to 91 kg N ha−1. Overall, the no-tillage and straw management had impact on some surface soil properties in sub-arctic Alaska, and no-tillage and minimum tillage increased barley crop yield.

Corn and Soybean Grain Phosphorus Content Relationship with Soil Phosphorus, Phosphorus Fertilizer, and Crop Yield

Most fertilizer phosphorus (P) rate recommendations for the north-central United States are based on combination of a critical soil-test P value and a mass-balance calculation of fertilizer P required to maintain critical soil-test P. Accurate estimates of grain P removal are an essential component of P mass-balance calculation. Current north-central extension service guidelines recommend that estimates of corn and soybean grain P removal should be calculated using constant grain P concentrations. We reviewed research from the north-central region to determine the extent to which variation in grain P concentration accounts for differences in crop P removal and to determine whether predictions of grain P concentration can be improved through consideration of soil-test P, crop yield, and fertilizer P application. We found that soil-test P, grain yield, and fertilizer P are predictor variables that may significantly improve estimates of grain P concentration for corn and soybeans.