Xiaobo Zhou

Sediment removal by prairie filter strips in row-cropped ephemeral watersheds

Twelve small watersheds in central Iowa were used to evaluate the effectiveness of prairie filter strips (PFS) in trapping sediment from agricultural runoff. Four treatments with PFS of different size and location (100% rowcrop, 10% PFS of total watershed area at footslope, 10% PFS at footslope and in contour strips, 20% PFS at footslope and in contour strips) arranged in a balanced incomplete block design were seeded in July 2007. All watersheds were in bromegrass ( L.) for at least 10 yr before treatment establishment. Cropped areas were managed under a no-till, 2-yr corn ( L.)-soybean [ (L.) Merr.] rotation beginning in 2007. About 38 to 85% of the total sediment export from cropland occurred during the early growth stage of rowcrop due to wet field conditions and poor ground cover. The greatest sediment load was observed in 2008 due to the initial soil disturbance and gradually decreased thereafter. The mean annual sediment yield through 2010 was 0.36 and 8.30 Mg ha for the watersheds with and without PFS, respectively, a 96% sediment trapping efficiency for the 4-yr study period. The amount and distribution of PFS had no significant impact on runoff and sediment yield, probably due to the relatively large width (37-78 m) of footslope PFS. The findings suggest that incorporation of PFS at the footslope position of annual rowcrop systems provides an effective approach to reducing sediment loss in runoff from agricultural watersheds under a no-till system.

Nutrient removal by prairie filter strips in agricultural landscapes

Nitrogen (N) and phosphorus (P) from agricultural landscapes have been identified as primary sources of excess nutrients in aquatic systems. The main objective of this study was to evaluate the effectiveness of prairie filter strips (PFS) in removing nutrients from cropland runoff in 12 small watersheds in central Iowa. Four treatments with PFS of different spatial coverage and distribution (No-PFS, 10% PFS, 10% PFS with strips, and 20% PFS with strips) were arranged in a balanced incomplete block design across four blocks in 2007. A no-tillage two-year corn (Zea mays L.) –soybean (Glycine max [L.] Merr.) rotation was grown in row-cropped areas beginning in 2007. Runoff was monitored by H flumes, and runoff water samples were collected during the growing seasons to determine concentrations of nitrate-nitrogen (NO3-N), total nitrogen (TN) and total phosphorus (TP) through 2011. Overall, the presence of PFS reduced mean annual NO3-N, TN, and TP concentrations by 35%, 73%, and 82%, respectively, and reduced annual NO3-N, TN, and TP losses by 67%, 84%, and 90%, respectively. However, the amount and distribution of PFS had no significant impact on runoff and nutrient yields. The findings suggest that utilization of PFS at the footslope position of annual row crop systems provides an effective approach to reducing nutrient loss in runoff from small agricultural watersheds.

Runoff and sediment yield under simulated rainfall on hillslopes in the Loess Plateau of China

Runoff volume, sediment yield and infiltration on hillslope in the hilly region of Loess Plateau were studied using a rainfall simulator. Two land cover (natural grassland, NG, and bare hillslope, BS) and three different rainfall intensities (2.0, 1.5, 0.75 mm min–1) were implemented. The runoff time of the NG2.0 treatment was 4.8 min earlier, the average infiltration rate was 5.60% lower, the runoff rate was 1.12 times higher and the sediment production rate was 3.28 times lower than those in the BS2.0 treatment. The runoff time for the NG1.5 and the NG0.75 treatments were 6 and 27 min slower, respectively, the average infiltration rate were 10.78 and 23.52% higher, respectively, the runoff rate were 1.67 and 4.25 times lower, respectively, and the sediment production rate were 5.21 and 16.75 times lower, respectively, than those for the BS1.5 and BS0.75 treatments. The effects of slope on sediment reduction were significantly greater than the effects of its runoff reduction, and there occurred more significant sediment-reducing effects with the decrease of rainfall intensity. Efficient use of rainfall runoff in the natural grassland at high intensity should be taken into considerations especially under current conditions of the co-existing drought-caused water shortage and soil erosion in the loess hilly regions.

Soil water during the drought of 2012 as affected by rye cover crops in fields in Iowa and Indiana

The drought of 2012 provides a unique opportunity to evaluate the effects of cover crop on soil moisture under relatively extreme conditions. The objective of this study was to quantify potential differences in soil moisture due to the presence of a rye (Secale cerale L.) cover crop in a corn (Zea mays L.)–soybean (Glycine max L.) rotation at various locations in the Midwestern United States during the drought of 2012. Soil volumetric water content (θ) and soil water storage (SWS) were monitored at three sites in Iowa and Indiana. Daily measurements of soil θ were taken at 10, 20, 40, and 60 cm (3.9, 7.9, 15.7, and 23.6 in) soil depths, and SWS was estimated to an 80 cm (31.5 in) depth. Soil water during the drought of 2012 was affected by a rye cover crop in comparison to without a rye cover crop for one (i.e., located in Iowa) of the three sites monitored. At the Iowa site, soil θ was on average 0.041 and 0.033 cm3 cm−3 (0.041 and 0.033 in3 in−3) greater at the 10 and 20 cm (3.9 and 7.8 in) depths, respectively, following termination of a rye cover crop than crops without a rye cover crop. Thus, during the 2012 drought, the use of a rye cover crop as compared to without a rye cover crop in a corn–soybean rotation did not significantly lower soil water conditions. The use of a cover crop either had no impact or significantly increased soil water conservation.

Subsurface Drainage Nitrate and Total Reactive Phosphorus Losses in Bioenergy-Based Prairies and Corn Systems

We compare subsurface-drainage NO-N and total reactive phosphorus (TRP) concentrations and yields of select bioenergy cropping systems and their rotational phases. Cropping systems evaluated were grain-harvested corn-soybean rotations, grain- and stover-harvested continuous corn systems with and without a cover crop, and annually harvested reconstructed prairies with and without the addition of N fertilizer in an Iowa field. Drainage was monitored when soils were unfrozen during 2010 through 2013. The corn-soybean rotations without residue removal and continuous corn with residue removal produced similar mean annual flow-weighted NO-N concentrations, ranging from 6 to 18.5 mg N L during the 4-yr study. In contrast, continuous corn with residue removal and with a cover crop had significantly lower NO-N concentrations of 5.6 mg N L when mean annual flow-weighted values were averaged across the 4 yr. Prairies systems with or without N fertilization produced significantly lower concentrations below <1 mg NO-N L than all the row crop systems throughout the study. Mean annual flow-weighted TRP concentrations and annual yields were generally low, with values <0.04 mg TRP L and <0.14 kg TRP ha, and were not significantly affected by any cropping systems or their rotational phases. Bioenergy-based prairies with or without N fertilization and continuous corn with stover removal and a cover crop have the potential to supply bioenergy feedstocks while minimizing NO-N losses to drainage waters. However, subsurface drainage TRP concentrations and yields in bioenergy systems will need further evaluation in areas prone to higher levels of P losses.

Comparing Nitrate Sink Strength in Perennial Filter Strips at Toeslopes of Cropland Watersheds

Integration of perennial filter strips (PFS) into the toeslopes of agricultural watersheds may decrease downstream nitrate (NO) losses. However, long-term NO removal depends on the relative importance of several NO sinks in the PFS. Plant biomass and labile soil organic matter (SOM) are temporary NO sinks, while stable SOM is a long-term, but potentially finite, NO sink. In contrast, denitrification is a permanent NO sink. We investigated the relative importance of these NO sinks in PFS at the toeslope of row crop watersheds in Iowa. Using 25- × 30-cm in situ mesocosms, we added NO to PFS soils and quantified NO-N recovery in plant biomass and SOM after one growing season. Further, we compared NO-N recovery in particulate (relatively labile) and mineral-associated (relatively stable) SOM in mesocosms with and without growing perennial vegetation. To determine the potential importance of denitrification, we compared denitrification enzyme activity in soils from paired watersheds with and without PFS. Transfer of NO-N into labile and stable SOM pools was rapid and initially independent of growing vegetation. However, SOM and plant biomass were both relatively minor NO sinks, accounting for <30% of NO-N inputs. Denitrification enzyme activity data indicated that dissolved organic carbon derived from perennial vegetation increased potential denitrifier activity in PFS soils compared with row crop soils. Together, these results constrain SOM and plant biomass as NO sinks and indicate that denitrification was the most important NO sink in perennial filter strips over one growing season.

Impacts of Incorporating Prairie Vegetation within Row Crop Production on Soil Hydraulic Properties

Runoff from agricultural land is a concern for downstream water quality. Soil hydraulic properties influence infiltration which influences surface runoff and, as a result, downstream water quality. Implementation of vegetative filter strips (VFS) has the potential to reduce downstream pollutant loading by slowing runoff velocities, which allows particulates to settle out, as well as allowing for infiltration. Since soil hydraulic properties influence infiltration there is a need to evaluate the impacts VFS have on physical properties of the soil, which will allow for a better understanding of the mechanisms by which VFS provide benefits. The objective of this study was to determine if differences in soil hydraulic properties exist under different land uses. Variations in surface infiltration between VFS, restored prairie, and agriculture row crop areas were determined utilizing tension infiltrometers for in-situ measurement of infiltration rate at the upslope and foot slope positions under various land cover in three small watersheds at the Neal Smith Wildlife National Refuge (NSNWR) near Prairie City, IA. Results did not show statistically significant differences in treatment at any of the tensions tested. There were significant differences in conductivity between the two landscape positions at tensions -6 & -12 cm. Although there were no significant differences collectively results did show higher conductivity within the VFS compared to the row crop and restored native prairie in two of three watersheds. Higher conductivity in the VFS of the two watersheds shows that over time VFS may influence soil hydraulic properties within a watershed. However the low conductivity in the restored native prairie does not seem to support the idea of long term effectiveness of VFS which warrant further investigation.