Susan J. Riha

U.S. agriculture and climate change: New results

We examined the impacts on U.S. agriculture of transient climate change assimulated by 2 global general circulation models focusing on the decades ofthe 2030s and 2090s. We examined historical shifts in the location of cropsand trends in the variability of U.S. average crop yields, finding thatnon-climatic forces have likely dominated the north and westward movement ofcrops and the trends in yield variability. For the simulated future climateswe considered impacts on crops, grazing and pasture, livestock, pesticide use,irrigation water supply and demand, and the sensitivity to international tradeassumptions, finding that the aggregate of these effects were positive for theU.S. consumer but negative, due to declining crop prices, for producers. Weexamined the effects of potential changes in El Niño/SouthernOscillation (ENSO) and impacts on yield variability of changes in mean climateconditions. Increased losses occurred with ENSO intensity and frequencyincreases that could not be completely offset even if the events could beperfectly forecasted. Effects on yield variability of changes in meantemperatures were mixed. We also considered case study interactions ofclimate, agriculture, and the environment focusing on climate effects onnutrient loading to the Chesapeake Bay and groundwater depletion of theEdwards Aquifer that provides water for municipalities and agriculture to theSan Antonio, Texas area. While only case studies, these results suggestenvironmental targets such as pumping limits and changes in farm practices tolimit nutrient run-off would need to be tightened if current environmentalgoals were to be achieved under the climate scenarios we examined

CARBON AND NUTRIENT ACCUMULATION IN SECONDARY FORESTS REGENERATING ON PASTURES IN CENTRAL AMAZONIA

Over the past three decades, large expanses of forest in the Amazon Basin were converted to pasture, many of which later degraded to woody fallows and were abandoned. While the majority of tropical secondary forest (SF) studies have examined post-deforestation or post-agricultural succession, we examined post-pasture forest recovery in 10 forests ranging in age from 0 to 14 years since abandonment. We measured above- ground biomass and soil nutrients to 45 cm depth and computed total site carbon (C) and nutrient stocks to gain an understanding of the dynamics of nutrient and C buildup in regenerating SF in central Amazonia. Aboveground biomass accrual was rapid, 11.0 Mg·ha 21 ·yr 21 , in the young SFs. Within 12-14 yr, they accumulated up to 128.1 Mg/ha of dry aboveground biomass, equivalent to 25-50% of primary forest biomass in the region. Wood nitrogen (N) and phosphorus (P) concentrations decreased with forest age. Aboveground P and calcium (Ca) stocks accu- mulated at a rate of 1.2 and 29.4 kg·ha 21 ·yr 21 ; extractable soil P stocks declined as forest age increased. Although soil stocks of exchangeable Ca (207.0 6 23.7 kg/ha) and extractable P (8.3 6 1.5 kg/ha) were low in the first 45 cm, both were rapidly translocated from soil to plant pools. Soil N stocks increased with forest age, probably due to N fixation, at- mospheric deposition, and/or subsoil mining. Total soil C storage to 45 cm depth ranged between 42 and 84 Mg/ha, with the first 15 cm storing 40-45% of the total. Total C accrual (7.04 Mg C·ha 21 ·yr 21 ) in both aboveground and soil pools was similar or higher than values reported in other studies. Tropical SFs regrowing on lightly to moderately used pasture rapidly sequester C and rebuild total nutrient capital following pasture abandonment. Translocation of some nutrients from deep soil (.45 cm depth) may be important to sustaining productivity and continuing biomass ac- cumulation in these forests. The soil pool represents the greatest potential for long-term C gains; however, soil nutrient deficits may limit future productivity.

Impact of temperature and precipitation variability on crop model predictions

Future climate changes, as well as differences in climates from one location to another, may involve changes in climatic variability as well as changes in means. In this study, a synthetic weather generator is used to systematically change the within-year variability of temperature and precipitation (and therefore also the interannual variability), without altering long-term mean values. For precipitation, both the magnitude and the qualitative nature of the variability are manipulated. The synthetic daily weather series serve as input to four crop simulation models. Crop growth is simulated for two locations and three soil types. Results indicate that average predicted yield decreases with increasing temperature variability where growing-season temperatures are below the optimum specified in the crop model for photosynethsis or biomass accumulation. However, increasing within-year variability of temperature has little impact on year-to-year variability of yield. The influence of changed precipitation variability on yield was mediated by the nature of the soil. The response on a droughtier soil was greatest when precipitation amounts were altered while keeping occurrence patterns unchanged. When increasing variability of precipitation was achieved through fewer but larger rain events, average yield on a soil with a large plant-available water capacity was more affected. This second difference is attributed to the manner in which plant water uptake is simulated. Failure to account for within-season changes in temperature and precipitation variability may cause serious errors in predicting crop-yield responses to future climate change when air temperatures deviate from crop optima and when soil water is likely to be depleted at depth.

A Farm-Level Analysis of Economic and Agronomic Impacts of Gradual Climate Warming

The potential economic and agronomic impacts of gradual climate warming are examined at the farm level. Three models of the relevant climatic, agronomic, and economic processes are developed and linked to address climate change impacts and agricultural adaptability. Several climate warming scenarios are analyzed, which vary in severity. The results indicate that grain farmers in southern Minnesota can effectively adapt to a gradually changing climate (warmer and either wetter or drier) by adopting later maturing cultivars, changing crop mix, and altering the timing of field operations to take advantage of a longer growing season resulting from climate warming.

Nutrient leaching in a Colombian savanna Oxisol amended with biochar.

Nutrient leaching in highly weathered tropical soils often poses a challenge for crop production. We investigated the effects of applying 20 t ha biochar (BC) to a Colombian savanna Oxisol on soil hydrology and nutrient leaching in field experiments. Measurements were made over the third and fourth years after a single BC application. Nutrient contents in the soil solution were measured under one maize and one soybean crop each year that were routinely fertilized with mineral fertilizers. Leaching by unsaturated water flux was calculated using soil solution sampled with suction cup lysimeters and water flux estimates generated by the model HYDRUS 1-D. No significant difference ( > 0.05) was observed in surface-saturated hydraulic conductivity or soil water retention curves, resulting in no relevant changes in water percolation after BC additions in the studied soils. However, due to differences in soil solution concentrations, leaching of inorganic N, Ca, Mg, and K measured up to a depth of 0.6 m increased ( < 0.05), whereas P leaching decreased, and leaching of all nutrients (except P) at a depth of 1.2 m was significantly reduced with BC application. Changes in leaching at 2.0 m depth with BC additions were about one order of magnitude lower than at other depths, except for P. Biochar applications increased soil solution concentrations and downward movement of nutrients in the root zone and decreased leaching of Ca, Mg, and Sr at 1.2 m, possibly by a combination of retention and crop nutrient uptake.

Regional yield estimation using a crop simulation model: Concepts, methods, and validation

Crop simulation models are increasingly being used in the analysis of regional agricultural production systems. The purpose of this study was to develop and test an approach to estimate regional crop yields using a crop simulation model. Potential average annual maize (Zea mays L.) yields for the period 1960–1989 in the Eastern crop reporting district of Illinois were estimated using a maize simulation model. Historical climate data were used, and representative soil series, crop varieties, and planting times for this region were considered in the simulations. The simulated potential yield was adjusted for economically efficient nitrogen use, disease, insect, and harvest losses to derive a final regional simulated yield. The simulated yield for each year of the study was compared to the historical average yield levels obtained by farmers in the region. To assess the value of incorporating varying levels of soil and planting date information in the model, four scenarios were considered: (1) one soil, one planting date; (2) three soils, one planting date; (3) one soil, seven planting dates, and (4) three soils, seven planting dates. The most detailed scenarios, (3) and (4), provided the best estimation of maize yields.

The Relationship between Extreme Hourly Precipitation and Surface Temperature in Different Hydroclimatic Regions of the United States

Abstract In a changing climate, there is an interest in predicting how extreme rainfall events may change. Using historical records, several recent papers have evaluated whether high-intensity precipitation scales with temperature in accordance with the Clausius–Clapeyron (C–C) relationship. For varying locations in Europe, these papers have identified both super C–C relationships as well as a breakdown of the C–C relationship under dry conditions. In this paper, a similar analysis is carried out for the United States using data from 14 weather stations clustered in four different hydroclimatic regions: the coastal northeast, interior New York, the central plains, and the western plains. In all regions except interior New York state, 99th percentile 1-h precipitation generally followed the C–C relation. In interior New York, there was evidence that intensity scaled with a super C–C relationship. For the 99.9th percentile precipitation, interior New York displayed some moderate evidence of a super C–C rela...

Relationships between soil hydrology and forest structure and composition in the southern Brazilian Amazon

Abstract Question: Is soil hydrology an important niche-based driver of biodiversity in tropical forests? More specifically, we asked whether seasonal dynamics in soil water regime contributed to vegetation partitioning into distinct forest types. Location: Tropical rain forest in northwestern Mato Grosso, Brazil. Methods: We investigated the distribution of trees and lianas ≥ 1 cm DBH in ten transects that crossed distinct hydrological transitions. Soil water content and depth to water table were measured regularly over a 13-month period. Results: A detrended correspondence analysis (DCA) of 20 dominant species and structural attributes in 10 × 10 m subplots segregated three major forest types: (1) high-statured upland forest with intermediate stem density, (2) medium-statured forest dominated by palms, and (3) low-statured campinarana forest with high stem density. During the rainy season and transition into the dry season, distinct characteristics of the soil water regime (i.e. hydro-indicators) were closely associated with each vegetation community. Stand structural attributes and hydro-indicators were statistically different among forest types. Conclusions: Some upland species appeared intolerant of anaerobic conditions as they were not present in palm and campinarana sites, which experienced prolonged periods of saturation at the soil surface. A shallow impermeable layer restricted rooting depth in the campinarana community, which could heighten drought stress during the dry season. The only vegetation able to persist in campinarana sites were short-statured trees that appear to be well-adapted to the dual extremes of inundation and drought. Nomenclature: Ribeiro et al. (1999).

COMPETITION FOR WATER IN A HEDGE-INTERCROP SYSTEM

Abstract Despite considerable interest in agroforestry systems, efforts to adopt these systems may be premature because it is unclear whether viable intercropping systems can be developed for environments where competition for limited resources, such as water, may be severe. Greater use of resources may occur in a mixed system compared to a monoculture, however, if the component species differ in root architecture. To investigate water use in a hedge-intercrop system, water uptake at different soil depths in a hedge ( Senna spectabilis )-annual (cowpea followed by maize) intercrop was measured and compared to uptake in monocultures and in soils where rooting depth was restricted by polyethylene mesh at 0.3 m depth. Hedge shoot pruning was an added experimental factor. Recharge of soil water below 0.45 m occurred only once during the cowpea season and once during the maize season. Annual crops in monoculture took up water at similar rates and depths as the multistem hedge monoculture and the intercropped systems. There was no evidence that root density restricted water uptake in the surface 0.45 m in any of the treatments and there was no increase in uptake below 0.45 m in intercropped treatments compared to monocrops. The yield of the annual crop intercropped with the hedge was less than half of the monoculture annual crop yield except where severe pruning delayed canopy development and hence water uptake by the hedge. Results indicate there may be little opportunity for increasing water uptake in hedge-intercrop systems in semi-arid environments.

Modeling the Impact of Natural Resource-Based Poverty Traps on Food Security in Kenya: The Crops, Livestock and Soils in Smallholder Economic Systems (CLASSES) Model

We investigate the interactions between natural resource-based poverty traps and food security for smallholder farms in highland Kenya using a recently developed system dynamics bio-economic model. This approach permits examination of the complex interactions and feedback between farm household economic decision-making and long-term soil fertility dynamics that characterize persistent poverty and food insecurity among smallholders in rural highland Kenya. We examine the effects of changing initial endowments of land and stocks of soil organic matter on smallholders’ well being, as reflected in several different indicators. We show that larger and higher quality land endowments permit accumulation of cash and livestock resources and conservation of soil organic matter relative to smaller or more degraded farms. This suggests the existence of asset thresholds that divide food secure households from food insecure ones.