Soil phosphorus loss in tile drainage water from long ...

Science of the Total Environment 580 (2017) 9–16

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Soil phosphorus loss in tile drainage water from long-term conventionaland non-tillage soils of Ontario with and without compost addition T.Q. Zhang a,⁎, C.S. Tan a, Y.T. Wang a, B.L. Ma b, T. Welacky a a b

Harrow Research and Development Centre, Agriculture and Agri-Food Canada, Harrow, ON N0R 1G0, Canada Ottawa Research and Development Centre, Agriculture and Agri-Food Canada, Ottawa, ON K1A 0C6, Canada

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• Quantification of agricultural practices on P loss is needed to reduce P loss risk. • Compost addition increased DRP loss at a rate 113% higher under NT than under CT. • DRP loss with NT was solely driven by DRP concentration in tile drainage water. • DRP loss with CT was collectively driven by DRP concentration and flow volume. • Compost addition did not affect PP loss, regardless of tillage practices.

a r t i c l e

i n f o

Article history: Received 7 October 2016 Received in revised form 1 December 2016 Accepted 2 December 2016 Available online xxxx Editor: Jay Gan Keywords: Agricultural management practice Surface water quality Organic amendments Sub-surface runoff Phosphorus Tile drainage

a b s t r a c t Recent ascertainment of tile drainage a predominant pathway of soil phosphorus (P) loss, along with the rise in concentration of soluble P in the Lake Erie, has led to a need to re-examine the impacts of agricultural practices. A three-year on-farm study was conducted to assess P loss in tile drainage water under long-term conventional(CT) and non-tillage (NT) as influenced by yard waste leaf compost (LC) application in a Brookston clay loam soil. The effects of LC addition on soil P loss in tile drainage water varied depending on P forms and tillage systems. Under CT, dissolved reactive P (DRP) loss with LC addition over the study period was 765 g P ha−1, 2.9 times higher than CT without LC application, due to both a 50% increase in tile drainage flow volume and a 165% increase in DRP concentration. Under NT, DRP loss in tile drainage water with LC addition was 1447 g P ha−1, 5.3 times greater than that for NT without LC application; this was solely caused by a 564% increase in DRP concentration. However, particulate P loads in tile drainage water with LC application remained unchanged, relative to non-LC application, regardless of tillage systems. Consequently, LC addition led to an increase in total P loads in tile drainage water by 57 and 69% under CT and NT, respectively. The results indicate that LC application may become an environmental concern due to increased DRP loss, particularly under NT. Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.

1. Introduction In the past few decades, mitigation of agricultural phosphorus (P) loss was mainly addressed by implementing practices (e.g. ⁎ Corresponding author. E-mail address: [email protected] (T.Q. Zhang).

http://dx.doi.org/10.1016/j.scitotenv.2016.12.019 0048-9697/Crown Copyright © 2016 Published by Elsevier B.V. All rights reserved.

conservation tillage) that are expected to reduce soil erosion and sediment-bound P loss (Kleinman et al., 2009). In the Great Lakes watersheds, not surprisingly, agricultural total P (TP) loads have often been found decreased mainly due to the reductions of particulate P (PP) losses (Richards et al., 2009; Daloğlu et al., 2012). However, the loads of dissolved reactive P (DRP), a form of P that is readily available to the aquatic biota, declined through the early 1990s, but then

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increased since the mid-1990s, as has the incidence of algal blooms (Daloğlu et al., 2012). Such increases in DRP loads and surface water deterioration may have been partly driven by the changes of agricultural management practices occurring in the last few decades (Michalak et al., 2013; Daloğlu et al., 2012). For example, increases of autumn fertilizer application, surface broadcast fertilizer application, and conservation tillage in the Maumee River region over the past ten years have been found to contribute to the observed 218% increase in DRP loadings into the river between 1995 and 2011 (Michalak et al., 2013). Therefore, re-emerging surface water quality degradation has raised a need to reexamine the impacts of agricultural management practices on agricultural P, particularly DRP, losses. Compared to conventional tillage (CT), non-tillage (NT) reduces soil erosion rates (mm yr−1) by 2.5 to N1000 times, with median and mean values of 20 and 488 times, respectively, according to 39 independent studies involving direct comparisons of soil erosion between CT and NT (Montgomery, 2007). Accordingly, PP loss from agricultural soils are also generally decreased, when NT is adopted (Ulén et al., 2010). In addition, NT practice is effective in controlling soil evaporation, improving soil structure, and reducing energy needs (Lai et al., 2007). With all these benefits, NT practice has been promoted rapidly around the world. In Canada, for instance, the percentage of NT land acreage in the total farm land prepared for seeding have increased to 56% in 2011 from 16% in 1996 (Census of Canadian Agriculture, 1996 and 2011). Meanwhile, it has become apparent that long-term NT practice tends to accumulate P in the upper few centimeters of soils, which may lead to an increase of DRP loss into surface water (Cade-Menun et al., 2010). On a poorly drained Brookston clay loam soil, for example, NT practice led to significantly higher DRP concentration in surface runoff (0.16–0.36 mg L−1 under CT vs. 0.41–1.88 mg L−1 under NT) and tile drainage water (0.20–0.30 mg L−1 under CT vs. 0.41–0.61 mg L−1 under NT) than CT (Gaynor and Findlay, 1995). One challenge facing crop production on many fine-textured soils is that average field-crop yields tend to plateau and perhaps even decline in some areas (Wallace and Terry, 1998). It is widely believed that intrinsically poor physical quality of fine-textured soils is one of the key factors leading to yield plateaus or decreases (Reynolds et al., 2003). If optimal soil physical quality could be achieved and maintained, average field-crop production could be increased by as much as 25–50% (Wallace and Terry, 1998). Recently, organic wastes (e.g., livestock manure and yard waste) have received renewed attention for their roles in improving soil physical quality of fine textured soils (Reynolds et al., 2003). For example, Reynolds et al. (2003) reported that yard waste compost improved organic C, bulk density, and plant-available water holding capacity of a Brookston clay loam soil. When organic wastes are used as soil conditioners to improve soil physical quality, amounts of P contained in the material are rarely balanced with regard to crop needs, and may exceed the requirement of crops, leading to increased P loss risk. However, limited information on effects of organic soil conditioner application on P loss is available under field conditions. Fine-textured soils generally have high potential for downward moving P loss due to greater likelihood of cracking and the subsequent preferential flow (Toor and Sims, 2015; van Es et al., 2004). In Ontario, over 70% of agricultural soils are tile drained, which would increase the risk of downward movement of P leaving farm fields. Subsurface drainage discharge contributed to 55–68% of total DRP loss from a Brookston clay soil during a three-year period under corn production (Gaynor and Findlay, 1995). Our recent study with a Perth clay soil also indicated that, under a corn-soybean rotation, tile drainage contributed to 95% of total DRP loads and 29–64% of PP loads (Tan and Zhang, 2011). Zhang et al. (2015) observed that P loss in tile drains from clay loam soils varied with cropping systems, but for any of which longterm continuous fertilizer P addition notably increased DRP and TP loss primarily caused by the increases in concentration of P. They further pointed out that P downward movement loss in tile drains must be taken into consideration from a water quality point of view.

Generally, NT practice along with application of organic amendment tends to improve water infiltration and further promote P transport deeper into soil profile (Stone and Schlegel, 2010; Hangen et al., 2002; Miller et al., 2002). Therefore, the knowledge of tillage practice and organic waste application on tile drainage P loss has particular significances on risk assessment of soil P loss from fine-textured soils. Also, it is essential to develop beneficial management practices that minimize P loss while maximizing crop production. For potential use of yard waste compost as a soil conditioner, researchers in Ontario, Canada, have had growing interest clarifying effects of yard waste compost application on soil health and P loss risk potential. Reynolds et al. (2003) reported that yard waste compost application improved soil physical conditions. The current study was conducted to further investigate effects of organic soil conditioner application on P loss in tile drainage water from a Brookston clay loam soil under long-term NT and CT practices. In our view, the results with yard waste compost may also provide indications for those with animal manure due to similarities between the both, which will be discussed in this paper. 2. Materials and methods 2.1. Experimental site and design The on-farm experiment was conducted at two farm fields, including Farm A (42° 12′ 15″ N, 82° 44′ 50″ W) and Farm B (42° 12′ 15″ N, 82° 45′ 58″W), from 15 Sept. 1998 to 14 Nov. 2001. The two farms located within 0.5 km of each other on a Brookston clay loam soil (Orthic Humic Gleysol). Brookston clay and clay loam soils are major agricultural soils in southwestern Ontario, occupying approximately 66% of the region's agricultural lands. The 45-year average (1961–2005) of annual precipitation was 831 mm in the experimental area. Farm A had been continued with NT practice since 1989, while farm B had been consistently under CT practice since 1991. Both farm fields were overall flat and had been applied with commercial fertilizer P at the rates locally recommended, as well as with sufficient N and K added to meet crop needs for essential nutrients. Other field and crop managements followed the local practices. Soil conditions would have been well established under each of the respective tillage systems, and would be spatially homogeneous in each individual farm field after approximate ten years of preparation prior to the formal treatments implemented. In addition, soil conditions associated with P loss risk seemed generally similar between the CT and NT farm fields, because P concentrations in tile drainage water were largely similar between CT and NT without LC compost application, which will be further discussed below. As such, the two farm fields provided an ideal and unique opportunity for this study to determine the effects of LC addition on soil P loss between soils under CT and NT. Each farm was divided into two portions (i.e. two plots) in the fall of 1998 shortly after crop harvesting, with one receiving LC and the other as control (i.e. zero-LC application). The portion areas for farm A under NT were 2.0 ha for LC and 2.4 ha for zero-LC, while for farm B under CT they were 2.4 and 2.2 ha for LC and zero-LC, respectively. Each plot on both NT and CT farms contained five subsurface tile drains (10.4-cm i.d.) spaced at 8.7 m, with an average depth of 0.6 m below the soil surface. The length of tiles was 538 m and 450 m at the NT and CT sites, respectively. 2.2. Crop management and compost application Cropping system on both field sites was a corn-soybean rotation, with soybean grown in 1999 and 2001 and corn grown in 2000. On May 7 and 12, 1999, soybeans were seeded at the rate of 566,500 seed ha− 1 on the CT site and at the rate of 579,040 seed ha−1 on the NT site, both with a row space of 38 cm. Between early May and early June in 2000, field corn was seeded (72,000 seed ha− 1) in 76.2 cm wide rows at both CT and NT sites. On May 4 and 12, 2001, soybeans

T.Q. Zhang et al. / Science of the Total Environment 580 (2017) 9–16

were seeded at the rate of 554,819 seed ha−1 on the CT site and at the rate of 579,000 seed ha−1 on the NT site, both with a row space of 38.1 cm. Yard waste leaf compost (LC) was selected for this study, because it has been proved to improve physical quality of a Brookston clay loam soil under Ontario conditions (Reynolds et al., 2003). The LC used in the study was derived from urban lawn cuttings, leaves, and chopped brush and was produced using a turned open-air windrow system (C/ N ratio, 15.5). In the compost plots, LC was applied at 75 Mg dry matter ha−1 on December 10, 1998, October 21, 1999, and December 8, 2000, respectively. The basic properties of the LC applied in 1998 and 1999 are given in Table 1, while the data for 2000 are not available, although the same procedures were followed for the preparation. The LC applied 1 +1 contained 152 and 71 kg ha− 1 bioavailable N (NO− 3 -N + NH4 N + 0.05% × organic N), 222 and 156 kg ha−1 total P, and 5.0 and 6.2 kg ha−1 water extractable P (WEP) in 1998 and 1999, respectively. A value of 0.05% is recommended to calculate the plant available N from organic N contained in solid organic waste with dry matter N50% (OMAFRA, 2009). Chemical fertilizers were applied at the locally recommended rates to both LC and zero-LC plots. Yard waste leaf compost and chemical fertilizers were broadcast to the plots on the same day each year under both CT and NT, but were then also incorporated for the plots under CT. Soils for the CT plots were mouldboard plowed to a depth of 20 cm each fall shortly after harvesting and disked in the early spring before planting. Crop residues were returned to the plots after harvesting under both CT and NT. All other soil and crop management followed the local practices.

2.3. Tile drainage flow monitoring, water sampling, and phosphorus determination Tile drainage water from each plot was discharged into a 2.3 m diameter by 4 m deep manhole located in a monitoring shed. Tile drainage flow volume was measured on a year-round continuous basis by using a calibrated tipping bucket system, as described by Tan et al. (2002). Tile drainage water samples were collected using the ISCO model 2900 automatic samplers (Lincoln, Nebraska, USA), which were activated by digital signals from the water meter, and programmed to take one sample for every 10,000 L on the CT farm site and 25,000 L on the NT farm site. The water samples were collected sequentially up to 24 samples over a period of time. The water samples were then transported to the laboratory, filtered through a 0.45-μm filter, and analyzed for dissolved reactive P (DRP). In order to evaluate the effects of LC addition on particulate (PP) and total P (TP) losses in tile drainage water, the unfiltered water samples collected in last two of the three-year study period were also analyzed for total P (TP) using the sulfuric acid-hydrogen peroxide digestion method (USEPA, 1983). The total dissolved P (TDP) of the filtered samples were determined using the acidified ammonium persulfate ((NH4)2S2O8) oxidation procedure (USEPA, 1983). Particulate P was calculated as the difference between TP and TDP. Phosphorus concentrations in all of the filtrates and digests were determined using a QuikChem Flow Injection Auto-Analyzer (Lachat Instruments, Table 1 Selected properties of the yard waste leaf compost applied in 1998 and 1999. Total N

Total P

Total K

Organic matter

NO3\ \N

NH4\ \N

Water extractable P

pH

0.471 0.0329

0.067 0.082

8.02 7.84

g kg−1 1998 1999

17.4 16

2.96 2.08

17.1 11

196 480

0.747 0.118

Total N, P, and K were determined using the concentrated sulfuric acid and hydrogen peroxide digestion method (Thomas et al., 1967); the pH was determined at the 1:10 ratio of leaf compost to distilled water (Atiyeh et al., 2000); organic matter was determined using the procedure of Sluiter et al. (2005).

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Milwaukee, WI) with the ammonium molybdate ascorbic acid reduction method of Murphy and Riley (1962).

2.4. Statistical analyses All statistical analyses were performed using the SAS procedures (SAS Institute Inc., 2009). The data were tested for normality, with natural logarithm transformations performed when necessary. Flow volume weighted mean (FWM) P concentrations were calculated for each sampling period. Also, the overall P concentration in tile drainage water over the three-year period was determined on a FWM basis. The differences in tile drainage flow volume, concentrations of FWM DRP, FWM PP, and FWM TP, and loads of DRP, PP and TP between LC and zero-LC application within each tillage system were evaluated by paired-t-test using a TTTEST procedure of SAS. In order to compare tile drainage flow volume between CT and NT, percentage of precipitation occurring as tile drainage was calculated to eliminate possible effects caused by variations in precipitation between the two sites. Similarly, the differences in percentage of precipitation occurring as tile drainage between CT and NT within either LC or the zero-LC treatment were also evaluated by the paired-t-test using a TTTEST procedure of SAS.

3. Results 3.1. Tile drainage flow volume The cumulative tile drainage flow volumes increased with time in an overall similar manner across both CT and NT systems, regardless of LC application (Fig. 1). Of the three-year study period, the cumulative tile drainage flow volumes showed subtle increases by 7.9 mm under CT and by 34.7 mm under NT during the first crop production cycle from April 27, 1999 to April 25, 2000, while it increased gradually over time for the rest of the time periods. Production of such a small amount of tile drainage flow may have been caused by the low precipitation during this period (i.e. 475 mm). The 45-year average (1961–2005) of annual precipitation was 831 mm in the area where the experimental sites were located, as mentioned previously. The response of tile drainage flow to LC application varied with tillage systems (Fig. 1). Yard waste leaf compost application significantly increased tile drainage flow volume, with an overall increase of 50% over the zero-LC application (468 mm for the LC plot vs. 313 mm for the zero-LC plot) for the entire study period (Table 2) under CT. Under NT, both LC and zero-LC treatments had similar tile drainage flow volumes, with each yielding a total of approximately 400 mm during the study period (Fig. 1 and Table 2). The percentages of precipitation occurring as tile drainage varied temporally and were up to 370% for the period of early spring, March– April, indicating a key role that snowmelt water played in the formation of tile drainage discharge (Fig. 2). Tile drainage water with zero-LC application accounted for a greater percentage of precipitation under NT (19.3%) than under CT (13.1%) during the study period (Table 2 and Fig. 2). This indicates that the soil under NT produced 47% more tile drainage flow discharge than under CT, when the same amount of precipitation fell into the plots without LC application. Tan et al. (2002) monitored tile drainage flow between May 1, 1995 and April 30, 2000, and concluded that 48% more tile drainage water was produced from the soil under NT than under CT in southwestern Ontario. However, with addition of LC, tile drainage removed largely similar percentages of precipitation water over the study period under both CT and NT (19.5% for CT vs. 18.3% for NT). The percentage of precipitation occurring as drainage calculated in our study is comparable with those reported by Gaynor and Findlay (1995), who found that tile drainage as a percentage of precipitation ranged from 16.4 to 24.5% in a Brookston clay loam soil.

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Fig. 1. Effects of yard waste leaf compost on cumulative tile drainage flow volume under conventional- (CT) and non-tillage (NT) practices in a Brookston clay loam soil.

Fig. 2. Effects of conventional- (CT) and non-tillage (NT) practices on percentage of precipitation occurring as tile drainage water in a clay loam soil with and without yard waste leaf compost application.

3.2. Dissolved reactive P loss Addition of LC significantly increased FWM DRP concentration in tile drainage water, regardless of tillage practice (Table 2 and Fig. 3). Such a DRP increase, furthermore, was greater in soils under NT than under CT. In the plots with zero-LC application, FWM DRP concentrations of tile drainage water were overall similar between CT and NT (0.062 mg L−1 for CT vs. 0.055 mg L−1 for NT, Table 2). With LC addition, FWM DRP concentrations in tile drainage water increased up to 0.365 mg L−1 under NT, 1.2 times higher than those under CT (0.164 mg L−1). Over the study period, LC treatment caused a total of 765 g DRP ha−1 lost in tile drainage water under CT, 2.9 times higher than did the zero-LC treatment (194 g DRP ha− 1), as a result of a 50% increase in tile drainage flow volume and a 165% increase in DRP concentration (Fig. 3 and Table 2). In comparison, there was an amount of 1447 g ha− 1 DRP lost from the soil amended with LC under NT during the study period, 5.3 times greater than did the zero-LC treatment (230 g

DRP ha− 1); however, such an increase was caused solely by the increases in DRP concentration. Over the three-year study period, increased DRP loss (i.e. the difference between LC and zero-LC) caused by LC application under NT was 113% higher than under CT. 3.3. Particulate P loss Application of LC to the soil under CT resulted in significantly lower FWM PP concentration (0.312 mg L−1) in tile drainage water than did the zero-LC addition (0.358 mg L−1). However, both treatments of LC and zero-LC produced similar cumulative PP loading over the threeyear study period, due to the greater tile drainage flow volume with LC under CT (Fig. 4 and Table 2). Under NT, however, no consistent patterns were observed over time about how LC applications impacted tile drainage FWM PP concentration (Fig. 4). According to Table 2, FWM PP concentrations in tile drainage water were overall similar between

Table 2 Tile drainage flow volume, percentage of precipitation occurring as tile drainage, flow weighted mean (FWM) concentration of dissolved reactive P (DRP), particulate P (PP), and total P (TP), and their loads in tile drainage water as related to yard waste leaf compost (LC) addition and tillage practice over the three-year study period.

Treatments

Tile flow

Percentage of flow in precipitation

(mm)

(%)

FWM DRP

FWM PP

FWM TP

(mg L−1)

DRP loads

PP loads

TP loads

(g ha−1)

Conventional tillage Zero-LC 313 a LC 468 b

13.1 a (a) 19.5 b (a)

0.062 a 0.164 b

0.358 a 0.312 b

0.503 a 0.563 a

194 a 765 b

869 a 1068 a

1222 a 1924 b

Non-tillage Zero-LC LC

19.3 a (b) 18.3 a (a)

0.0549 a 0.365 b

0.330 a 0.344 a

0.445 a 0.878 b

230 a 1447 b

1085 a 967 a

1462 a 2475 b

418 a 396 a

Within each tillage system, means with the same letter in columns are statistically identical at P ≤ 0.05. For the percentage of precipitation occurring as tile drainage water, the differences between CT and NT were also tested within the LC treatment or within the zero-LC treatment; means with the same letter in the parentheses are statistically identical at P b 0.05 between CT and NT.

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Fig. 3. Effects of yard waste leaf compost on flow volume weighted mean (FWM) of dissolved reactive phosphorus (DRP) concentration and cumulative DRP loads in tile drainage water under conventional tillage (CT) and non-tillage (NT) practices in a Brookston clay loam soil. Vertical bars are standard errors.

zero-LC and LC treatments (0.33 mg L−1 vs. 0.34 mg L−1) under NT. With similar tile drainage flow volume between both treatments under NT, it is not surprised that both the zero-LC and LC treatments yielded similar cumulative PP loading during the study period (Table 2 and Fig. 4). 3.4. Total P loss Compared to the zero-LC treatment, annual LC application, in general, did not have detectable effects on FWM TP concentration in drainage water under CT during the study period (Fig. 5 and Table 2). With increased tile drainage flow volume, addition of LC to soils under CT increased TP loading in tile drainage water from 1.22 kg ha − 1 to 1.92 kg ha − 1, an equivalent of 57% increase, compared with zero-LC (Table 2). Under NT, FWM TP concentration in tile drainage water increased by 97% with LC addition, causing an overall increase of TP loading by 69%, relative to the zero-LC treatment. Hence, LC treatment under NT produced greater tile drainage TP loading than did the zero-LC treatment, though both treatments had similar tile drainage flow volume (Table 2). 4. Discussion Nutrients contained in LC (i.e., 16–17.4 g total N kg−1, 2.08–2.96 g total P kg−1, and 11–17.1 g total K kg−1) were reasonably consistent with the values reported for solid cattle manure (i.e. 16.7–27.1 g total N kg−1; 2.65–7.62 g total P kg−1; 13.6–22.9 g total K kg−1) collected in Ontario (OMAFRA, 2009; Wang et al., 2016). The pH (7.8–8.0) of LC was comparable with those (i.e. N 7) for liquid swine, liquid dairy, and solid beef manures measured in our recent study (Wang et al., 2016).

Organic matter content of LC (196–480 g kg−1) was also comparable with the value of 396 g kg−1 reported for cattle manure (Moral et al., 2005). These similarities between LC and animal manures, particularly cattle manure, suggest that the results from our current study may also have indications for the impacts of cattle manure application on surface water quality. It was reported that cattle manure accounts for approximately 63% of the total volume of manure produced annually in Ontario (Fraser et al., 2006). One difference between LC and animal manure is that the portion of mineral N (i.e. 0.9–7.0%) in total N was lower in LC than those reported for animal manure (OMAFRA, 2009). For instance, NH+1 4 -N alone accounts for 6–67% of total N contained in manures produced in Ontario (OMAFRA, 2009). With the assumption that mineralization rates of organic N are similar between LC and animal manure, one may reasonably conclude that amount of N available to crop growth and transport to surface water is lower with LC than with animal manure, if identical amount of organic N is applied. Another difference between LC and animal manure is that the percentage of total P as WEP was only 2.3–3.9% for LC, far below those (i.e. 12.5–22.3%) reported for major types of manures in Ontario (Wang et al., 2016). Water extractable P is often positively related to dissolved P loss in surface runoff from the soil recently amended with organic waste (Kleinman et al., 2005; Sharpley and Moyer, 2000). Hence, soils amended with LC may have lower risk for P loss than those amended with manures, particularly within a short time period after application. Addition of soil conditioners is aimed to improve physical quality of the soil, without much consideration of the need of crops for essential nutrients. The consequence for this is that amount of total N and P applied in organic soil conditioners would possibly exceed the requirement of crops for N and P, and cause increase of the risk for N and P loss in runoff water. From an

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Fig. 4. Effects of yard waste leaf compost on flow volume weighted mean (FWM) of particulate phosphorus (PP) concentration and cumulative PP loads in tile drainage water under conventional tillage (CT) and non-tillage (NT) practices in a Brookston clay loam soil. Vertical bars are standard errors.

environmental perspective, therefore, LC as a soil conditioner may be preferred to animal manures. Both CT and NT farms selected for this study had similar background risk potential for soil P loss. This was reflected by the FWM DRP, FWM PP, and FWM TP concentrations in tile drainage water, that were largely similar between CT and NT plots when zero-LC was added. In our view, such similarities provided a valid starting point to compare NT with CT practices regarding to their tile drainage P losses as influenced by LC application. The data from the zero-LC plots support that tile drainage flow volume often increases with the adoption of NT (King et al., 2015). Relative to CT, there are generally well-developed and continuous macro-pores (e.g., root channels and earthworm burrows) in soils under NT, which would contribute to preferential flow and thus increase tile drainage flow volume (Stone and Schlegel, 2010; Hangen et al., 2002). In addition, crop residues remaining on the soil surface under NT tend to protect soil from raindrop impact, thereby reducing crusting and sealing and favoring the downward flow of water in the soil profile (Stone and Schlegel, 2010). The results from this study showed that response of tile drainage flow volume to LC addition varied with tillage systems. In the soils under either CT or NT, LC application increased DRP concentration in tile drainage water, which was in consistent with the studies reported by Hergert et al. (1981) and McDowell and Sharpley (2001). The current study further disclosed that the soils under NT tended to lose more DRP in tile drainage water than under CT, when LC was applied annually. Under NT, P, that is applied in LC and released from crop residues, tends to remain on the surface of soil due to lack of mixing with soil components (Cade-Menun et al., 2010). In contrast, under CT, soil

cultivation can effectively mix P in the surface layer and break down P stratification, which would enhance the reactions of applied P with the soil components and reduce P that is available to losses in field water discharge (Pierce, 1994; Sharpley, 2003). Generally, levels of available P in surface soil are highly related to soil P losses in surface and subsurface runoff (Wang et al., 2010, 2012; Zhang et al., 2015). Therefore, it is not surprising to observe increased DRP concentration in tile drainage water from the soils receiving P under NT than under CT. Generally, PP loss in runoff water is highly dependent on soil sediment loss (Richards et al., 2009). Application of organic wastes, such as manure and compost, increases soil aggregation stability, water infiltration rate, macro-porosity, and hydraulic conductivity, due to addition of organic C (Boyle et al., 1989; Aggelides and Londra, 2000; Whalen et al., 2003; Celik et al., 2004). All these changes in soil properties tend to promote downward movement of water in soil profile and to reduce soil erosion. Hence, it is reasonable to observe the increased tile drainage flow volume along with the decreased PP concentration with LC treatment under CT. In fact, it has been widely reported that manure addition often cause a reduction in surface runoff volume and in sediment loss (Spargo et al., 2006; Wortmann and Walters, 2006; Ginting et al., 1998). However, we did not observe detectable differences in tile drainage flow volume and PP concentration between LC and zero-LC treatments under NT. There are generally low risks for sediment loss and thus PP loading from soils under NT, due to the lack of soil disturbance and the improved soil physical properties associated with organic C increase in the surface layer of soil. Also, soils under NT often yield higher tile drainage flow volume than those under CT, as discussed previously. The results from the current study indicate that LC application would not further increase tile drainage flow volume and decrease PP

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Fig. 5. Effects of yard waste leaf compost on flow volume weighted mean (FWM) of total phosphorus (TP) concentration and cumulative TP loads in tile drainage water under conventional tillage (CT) and non-tillage (NT) practices in a Brookston clay loam soil. Vertical bars are standard errors.

concentration in soils under NT, which already have high background potential to promote water infiltration and reduce soil sediments. Similar phenomena occurred to surface runoff (Mueller et al., 1984). During a surface runoff event, manure application decreased soil sediment concentration from 7.12 g L−1 to 3.86 g L−1 under CT, while under NT both manured and non-manured soils had similar and low sediment concentration (0.60–0.85 g L−1) (Mueller et al., 1984). Obviously, the results from the current study indicate that application of organic amendment for improvement of physical properties may become an environmental concern, due to the increased P loss, particularly DRP loss, in tile drainage water. Though LC application reduced PP concentration in tile drainage water under CT, this benefit could not compensate the adverse effects of LC application on surface water quality. Firstly, decreased PP concentration did not cause a reduction in PP loads, as tile drainage flow volume increased with LC application. Secondly, LC application increased FWM DRP concentration by 160%, while it decreased FWM PP concentration only by 13% (Table 2). Currently, NT has been promoted across Canada and around the world as a beneficial management practice to reduce agricultural contribution to sediment and sediment-bound nutrient loading (Tiessen et al., 2010). Compared to soils under CT, however, those under NT had high risk for losing additional DRP, due to both increased concentration of DRP and production of tile drainage flow, when organic amendments were applied. Some evidence did indicate that the adoption of NT and P application are two of the important drivers to increase DRP loading in agricultural tributaries of the Lake Erie observed since the mid1990s (Daloğlu et al., 2012). Thus, our study emphasizes the need to reduce DRP losses from soils under NT to surface water by adopting appropriate agricultural management practices. Currently, modified NT (e.g. rotational tillage), with which the soils are tilled every other year,

instead of continuous NT practices, is increasingly a common practice in Ontario and in the central-corn belt of the USA. More research is needed to investigate impacts of modified NT on DRP losses. In addition, future research needs to develop new soil P testing and to optimize P application timing, rates, and methods to achieve reduction of DRP losses in tile drainage water from soils under NT to surface water. 5. Conclusions Our study indicates that various tillage systems had differential risk for P loss after land application of organic amendments. Compared to zero-LC addition, LC application increased DRP loss in tile drainage water, regardless of tillage systems. Such an increase in DRP loss, however, was approximately 113% greater under NT than under CT. The increased DRP loss from the soils under NT was solely caused by a 564% increase in DRP concentration in tile drainage discharge, while those from the soils under CT were a result of a 50% increase in tile drainage flow volume and a 165% increase in DRP concentration in tile drainage water. In our study, LC application did not cause obvious changes in PP loss, regardless of tillage practices. The results clearly show that the soils under NT had higher risk of losing DRP in tile drainage water, when organic waste was applied. To mitigate increases in DRP losses, therefore, appropriate agricultural management practices must be developed and implemented on soils under NT. Acknowledgement Gratitude is expressed for technical assistance provided by M. Soutani, B. Hohner, M.R. Reeb, and K. Rinas from the Harrow Research and Development Center, Agriculture and Agri-Food Canada (AAFC).

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