Determining the Effect of Drying Time on Phosphorus Solubilization from Three Agricultural Soils under Climate Change Scenarios

Investigating water quality sensitivity to climate variability and its influencing factors in four Lake Erie watersheds

Climate change alters weather patterns and hydrological cycle, thus potentially aggravating water quality impairment. However, the direct relationships between climate variability and water quality are complicated by a multitude of hydrological and biochemical mechanisms dominate the process. Thus, little is known regarding how water quality responds to climate variability in the context of changing meteorological conditions and human activities. Here, a longitudinal study was conducted using trend, correlation, and redundancy analyses to explore stream water quality sensitivity to temperature, precipitation, streamflow, and how the sensitivity was affected by watershed climate, land cover percentage, landscape configuration, fertilizer application, and tillage types. Specifically, daily pollutant concentration data of suspended solid (SS), total phosphorus (TP), soluble reactive phosphorus (SRP), total Kjeldahl nitrogen (TKN), nitrate and nitrite (NO_x), and chloride (Cl) were used as water quality indicators in four Lake Erie watersheds from 1985 to 2017, during which the average temperature has increased 0.5 °C and the total precipitation has increased 9%. Results show that precipitation and flow were positively associated with SRP, NO_x, TKN, TP, and SS, except for SRP and NO_x in the urban basin. The rising temperatures led to increasing concentrations of SS, TKN, and TP in the urban basin. SRP and NO_x sensitivity to precipitation was higher in the years with more precipitation and higher precipitation seasonality, and the basins with more spatially aggregated cropland. No-tillage and reduced tillage management could decrease both precipitation and temperature sensitivity for most pollutants. As one of the first studies leveraging multiple watershed environmental variables with long-term historical climate and water quality data, this study can assist target land use planning and management policy to mitigate future climate change effects on surface water quality.

Tackling unintended consequences of grazing livestock farming: Multi-scale assessment of co-benefits and trade-offs for water pollution mitigation scenarios

A farm-to-landscape scale modelling framework combining regulating services and life cycle assessment mid-point impacts for air and water was used to explore the co-benefits and trade-offs of alternative management futures for grazing livestock farms. Two intervention scenarios were compared: one using on-farm interventions typically recommended following visual farm audits (visually-based) and the other using mechanistical understanding of nutrient and sediment losses to water (mechanistically-based). At farm scale, reductions in business-as-usual emissions to water of total phosphorus (TP) and sediment, using both the visually-based and mechanistically-based scenarios, were <5%. These limited impacts highlighted the important role of land drains and the lack of relevant on-farm measures in current recommended advisory lists for the soil types in question. The predicted impacts of both scenarios on free draining soils were significantly higher; TP reductions of ∼9% (visually-based) and ∼20% (mechanistically-based) compared with corresponding respective estimates of >20% and >35% for sediment. Key co-benefits at farm scale included reductions in nitrous oxide emissions and improvements in physical soil quality, whereas an increase in ammonia emissions was the principal trade-off. At landscape scale, simulated reductions in business-as-usual losses were <3% for both pollutants for both scenarios. The visually-based and mechanistically-based scenarios narrowed the gaps between current and modern background sediment loads by 6% and 11%, respectively. The latter scenario also improved the reduction of GWP100 relative to business-as-usual by 4%, in comparison to 1% for the former. However, with the predicted increase of ammonia emissions, both eutrophication potential and acidification potential increased (e.g., by 7% and 14% for the mechanistically-based scenario). The discrepancy of on-farm intervention efficacy across spatial scales generated by non-agricultural water pollutant sources is a key challenge for addressing water quality problems at landscape scale.

Alternate wetting and drying irrigation increases water and phosphorus use efficiency independent of substrate phosphorus status of vegetative rice plants

Sustainable approaches to rice cultivation that apply less irrigation and chemical fertilisers are required to increase crop resource use efficiency. Although alternate wetting and drying (AWD) has been widely promoted as a water-saving irrigation technique, its interactions with phosphorus (P) nutrition have attracted little attention. Vegetative rice plants were grown with two phosphorus levels, fertilised (HP) or un-fertilised (LP), and either continuous flooding (CF) or AWD irrigation. Treatment effects on substrate P bioavailability (measured by Diffusive Gradients in Thin films - DGT-P), plant and substrate water relations, and foliar phytohormone status, were assessed along with P partitioning in planta. Shoot biomass and leaf area under different irrigation treatments depended on substrate P status (significant P x irrigation interaction), since LP decreased these variables under CF, but had no significant effect on plants grown under AWD. AWD maintained DGT-P concentrations and increased maximal root length, but decreased root P concentrations and P offtake. Substrate drying decreased stomatal conductance (g_s) and leaf water potential (Ψ_leaf) but re-flooding increased g_s. AWD increased foliar abscisic acid (ABA), isopentenyl adenine (iP) and 1-aminocyclopropane-1-carboxylic acid (ACC) concentrations, but decreased trans-zeatin (tZ) and gibberellin A1 (GA1) concentrations. Low P increased ACC and jasmonic acid (JA) concentrations but decreased gibberellin A4 (GA4) concentrations. Across all treatments, stomatal conductance was negatively correlated with foliar ABA concentration but positively correlated with GA1 concentration. Changes in shoot phytohormone concentrations were associated with increased water and phosphorus use efficiency (WUE and PUE) of vegetative rice plants grown under AWD.

Phosphorus leaching from riparian soils with differing management histories under three grass species

Plants release carbon-based exudates from their roots into the rhizosphere to increase phosphorus (P) supply to the soil solution. However, if more P than required is brought into solution, additional P could be available for leaching from riparian soils. To investigate this further, soil columns containing a riparian arable and buffer strip soil, which differed in organic matter contents, were sown with three common agricultural and riparian grass species. The P loads in leachate were measured and compared with those from unplanted columns, which were 0.17 ± 0.01 and 0.89 ± 0.04 mg kg^-1 for the arable and buffer strip soil, respectively. A mixture of ryegrass and red fescue significantly (p ≤ .05) increased dissolved inorganic P loads in leachate from the arable (0.23 ± 0.01 mg kg^-1 ) and buffer strip soil (1.06 ± 0.05 mg kg^-1 ), whereas barley significantly reduced P leaching from the buffer strip soil (0.53 ± 0.08 mg kg^-1 ). This was dependent on the dissolved organic C released under different plant species and on interactions with soil management history and biogeochemical conditions, rather than on plant uptake of P and accumulation into biomass. This suggested that the amount and forms of P present in the soil and the ability of the plants to mobilize them could be key factors in determining how plants affect leaching of soil P. Selecting grass species for different stages of buffer strip development, basing species selection on root physiological traits, and correcting soil nutrient stoichiometry in riparian soils through vegetative mining could help to lower this contribution.

Soil Phosphorus Modeling for Modern Agriculture Requires Balance of Science and Practicality: A Perspective

The use of phosphorus (P) fertilizers in arable crop and pastoral systems is expected to change as modern agriculture is challenged to produce more food with fewer inputs. Agricultural systems models offer a dual purpose to support and integrate recent scientific advances and to identify strategies for farmers to improve nutrient efficiency. However, compared with nitrogen and carbon, advances in P modeling have been less successful. We assessed the potential opportunity of P modeling to increase P efficiency for modern agriculture and identified the current challenges associated with modeling P dynamics at the field scale. Three major constraints were (i) a paucity of detailed field datasets to model strategies aimed at increasing P use efficiency, (ii) a limited ability to predict P cycling and availability under the local effects of climate change, and (iii) a restricted ability to match measured soil P fractions to conceptual and modelable pools in soils with different mineral properties. To improve P modeling success, modelers will need to walk a tightrope to balance the roles of assisting detailed empirical research and providing practical land management solutions. We conclude that a framework for interdisciplinary collaboration is needed to acquire suitable datasets, continually assess the need for model adjustment, and provide flexibility for progression of scientific theory. Such an approach is likely to advance P management for increased P use efficiency.