Effect of cover crops on leaching of dissolved organic nitrogen and carbon in a maize-cover crop rotation in Mediterranean Central Chile

Agricultural Water Management 212 (2019) 399–406

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Effect of cover crops on leaching of dissolved organic nitrogen and carbon in a maize-cover crop rotation in Mediterranean Central Chile

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Osvaldo Salazara, , Liliana Balboab, Kiri Peraltab, Michel Rossib, Manuel Casanovaa, Yasna Tapiaa, Ranvir Singhc, Miguel Quemadad a

Departamento de Ingeniería y Suelos, Facultad de Ciencias Agronómicas, Universidad de Chile, Casilla 1004, Santiago, Chile Programa de Magíster en Manejo de Suelos y Aguas, Universidad de Chile, Casilla 1004, Santiago, Chile c Institute of Agriculture and Environment, Massey University, Palmerston North 4410, New Zealand d Departamento Producción Agraria, Universidad Politécnica de Madrid, 28040, Spain b

A R T I C LE I N FO

A B S T R A C T

Keywords: Agriculture Water quality Diffuse pollution Soil percolation Nitrogen and carbon loads

Protection and management of water quality across agricultural landscapes requires a sound understanding of runoff and/or leaching of nutrients and other agrichemicals from agricultural production systems to receiving waters. We, in a large leaching columns experiment, studied the losses of dissolved organic N (DON), dissolved organic carbon (DOC), dissolved inorganic N (DIN) and total dissolved N (TDN) from maize cultivation on a coarse-textured soil in in Mediterranean Central Chile. The combined effects of cover crops and inorganic N fertilisation rates were evaluated on nitrogen and carbon leaching loads (DIN, DON and DOC) and ratios of soluble components (DON:DIN, DON:TDN and DOC:DON). A total of 52 soil columns for 13 treatments (4 replicates) were established to evaluate leaching of dissolved N and C forms from: 1) continuous bare soil (fallow) compared with a continuous cover crop (Lolium multiflorum or Trifolium repens), with 0 or 150 kg N ha–1 applied; and 2) maize-fallow and maize-cover crop rotations with two different N doses (250 or 400 kg N ha–1) for the maize and cover crops (L. multiflorum and/or T. repens). We found that inclusion of a grass cover crop (L. multiflorum) and optimal N fertilisation (250 kg N ha-1) treatment resulted into lower DIN losses from the study columns. However, in trial 1, the DON load from the treatments with continuous grass cover crop L. multiflorum was on average twice the DIN load. In trial 2, the crop rotation of maize cultivation with 400 kg N ha–1 applied and inclusion of a legume cover crop T. repens resulted into the highest DIN loads, while a crop rotation of maize with 250 kg N ha–1 applied and inclusion of a grass cover crop L. multiflorum had the lowest DIN loads. However, the latter rotation gave significantly higher DON loads than the maize-fallow treatments. In trials 1 and 2, inclusion of L. multiflorum enhanced soil organic pools and microbial activity, and thus increased the amount of DON and DOC susceptible to leaching. Overall, the rotation with maize with 250 kg N ha–1 applied and L. multiflorum as cover crop generated the lowest amount of TDN leaching from the soil columns. We recommend this to be further studied in field conditions as a best management practice for reducing the risk of diffuse pollution of surface water bodies and groundwater from maize cultivation in Mediterranean Central Chile.

1. Introduction Protection and management of water quality across agricultural landscapes requires a sound understanding of runoff and/or leaching of nutrients and other agrichemicals from agricultural production systems to receiving waters. Dissolved inorganic nitrogen (DIN) forms, such as nitrate (NO3-N) and ammonium (NH4-N), have been identified as the main forms of nitrogen (N) leaching in agricultural systems (Galloway et al., 2008; Vitousek et al., 2009; Salazar et al., 2013). However, recent studies include dissolved organic N (DON) leaching as another

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important N loss pathway from agroecosystems (Abaas et al., 2012; McGovern et al., 2014; Scott and Rothstein, 2014). According to van Kessel et al. (2009), it has been known for more than 125 years that DON leaching losses occur from agricultural fields, but most N loss studies on agricultural systems have not measured DON as a potential pathway. Jones et al. (2004) concluded that DON is an important soluble N pool within total dissolved nitrogen (TDN) in soil and, although its full ecological significance remains unknown, they recommend measuring DON on a routine basis alongside measurements of DIN losses from agricultural soils. Dissolved organic N losses could explain a

Corresponding author. E-mail address: [email protected] (O. Salazar).

https://doi.org/10.1016/j.agwat.2018.07.031 Received 7 April 2018; Received in revised form 25 July 2018; Accepted 26 July 2018 Available online 21 September 2018 0378-3774/ © 2018 Elsevier B.V. All rights reserved.

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large fraction of the ‘missing nitrogen’ in N budgets (Pujo-Pay and Conan, 2003). For example, van Kessel et al. (2009) and Manninen et al. (2018) reported that, on average, DON leaching losses are equivalent to one-third of total dissolved N (TDN) losses from cultivated soil including grassland, pasture and cereals from Europe. Recently, several studies have been carried out investigating leaching of DON from soils under forest (Kiikkilä et al., 2013; Campbell et al., 2014) and grassland (Necpalova et al., 2012; Riaz et al., 2012; Hoeft et al., 2014). However, only a few studies has focused on investigating losses of DON from arable soils (Macdonald et al., 2016; Manninen et al., 2018), particularly in the Mediterranean climatic conditions. To the authors knowledge, there are few studies published to date investigating DON content on soils under Mediterranean conditions, for instance the studies carried out in plant communities in Spain (Delgado-Baquerizo et al., 2011) and vineyard in Greece (Christou et al., 2006). Neff et al. (2003) noted that DON contains a mixture of recalcitrant and labile compounds with substantial and very different roles in terrestrial biochemistry, which may affect the N balance in soil in two ways. Firstly, it can enable a ‘short-circuit’ in the terrestrial N cycle whereby plants absorb some DON forms directly from the soil solution, without the need for microbial mineralisation in soil (Näsholm et al., 2009). Secondly, it may represent a significant N loss when plants and edaphic microorganisms cannot assimilate available DON in the soil solution, with some forms of DON being flushed from ecosystems during rapid N leaching (Neff et al., 2003). As mentioned above, the latter has been widely demonstrated for forest (Kiikkilä et al., 2013; Campbell et al., 2014) and grassland ecosystems (Necpalova et al., 2012; Riaz et al., 2012; Hoeft et al., 2014), but it has not been properly investigated and quantified in agricultural soils. Thus, it is important to evaluate the potential effects on DON pools and crop production caused by excessive DON leaching in agricultural soils. For instance, Ros et al. (2009) noted that after N fertiliser application to soils, changes in N flux can occur through the DON pools. However, there are still many gaps in our understanding of the significance of DON in agricultural production systems as a potential pathway for N losses (Murphy et al., 2000). The impact of DON losses from agricultural soils on water quality has recently been shown in different studies where DON in surface waters was related to the surrounding area of agricultural land (Rasmussen et al., 2008; Bartley et al., 2012; Wohlfart et al., 2012; Evans et al., 2014). At the catchment level, agricultural fields have been linked to increase DON export (Mattsson et al., 2009), with the increase suggested to originate from intensive farming practices (Mattsson et al., 2005). Van Kessel et al. (2009) concluded that leaching of DON from agricultural soils into surface water leads to eutrophication and acidification. On the other hand, Worrall et al. (2004) noted that the removal of DOC from water sources represents one of the major costs to water treatment in large parts of Britain. Therefore, a better understanding and quantification of DON and DOC leaching from agricultural soils would help minimise its losses and effects on agricultural production and receiving water quality. In the Mediterranean zone of Chile, there is a serious risk of diffuse pollution of surface water bodies and groundwater due to excessive applications of N fertilizers for maize (Zea mays) under irrigation (Fuentes et al., 2014; Corradini et al., 2015; Nájera et al., 2015). This is a particular concern in areas with coarse-textured soils, which are more prone to N leaching due to low water-holding capacity (Casanova et al., 2013). These concerns also extend to other Mediterranean climate regions of the world where irrigated maize fields with high N doses represent a high risk of creating diffuse N pollution areas (Isidoro et al., 2006; Berenguer et al., 2009; Salmerón et al., 2011; Simeonova et al., 2017). Establishment of cover crops during the intercropping period of maize (replacing bare fallows) in these Mediterranean zones has been proposed to counteract the negative impacts of DIN leaching or diffuse pollution from irrigated maize fields (Salmerón et al., 2011; Gabriel and Quemada, 2011; Gabriel et al., 2012). However, most soils in such

agroecosystems are depleted of their antecedent soil organic carbon (SOC) pool and inclusion of cover crops can enhance SOC pool and microbial activity (Lal, 2013), hence increasing the risk of DON leaching due to higher total inputs of N (van Kessel et al., 2009). Another important consideration is that cover crop species (e.g. grasses (Poaceae) or legumes (Fabaceae)) used in a crop rotation may influence the DON:DIN ratio, but their impact on DON leaching is particularly poorly understood. Moreover, because plant residues from a winter legume included as a cover crop are rapidly decomposed (Wagger et al., 1998), it is well known that this may increase SOC and organic N. There is a risk of this organic N leaching from the system if its release from cover crop residues and the N organic mineralisation is not synchronised with the N requirements of the following crop (Gentry et al., 2013). However, the components of dissolved organic matter (DOM), DON and dissolved organic carbon (DOC) are directly associated in soilwater systems. Vinther et al. (2006) reported higher losses of DOC by leaching from cover crops than from bare soil in Denmark. Vinther et al. (2006) also noted that DOC could be an important energy source for denitrifying bacteria in deeper soil layers and thereby reduce leaching of dissolved N forms to groundwaters. In addition, there are potential effects of leached DOC on receiving water quality due to changes in the functioning of aquatic ecosystems, through its influence on light regime, energy and nutrient supply, and metal toxicity (Evans et al., 2005). There are very limited understanding and quantification of the potential effects of cover crops on cycling and losses of DON and DOC from soils under maize cultivation under Mediterranean conditions. In this study, we examined the combined effects of inorganic N fertilisation and cover crop inclusions on nitrogen and carbon leaching loads (DIN, DON and DOC) and ratios of soluble components (DON:DIN, DON:TDN and DOC:DON) from a coarse-textured soil from Central Chile, to reduce their impact on receiving water quality across agricultural landscape in Central Chile and similar Mediterranean conditions in other places worldwide. 2. Material and methods 2.1. Experiment description The study was conducted in a temperature-controlled glasshouse (25 °C), on undisturbed soil columns packed in PVC tubes (0.2 m diameter, 0.5 m long), at the Antumapu Experimental Station located in Santiago, Chile (33°34′S, 70°38′W). The soil was taken from the Antumapu Experimental Station and is classified as an Entic Haploxeroll (CIREN, 1996). A total of 52 (13 treatments * 4 replicates) soil columns were established and monitored over a period of August 2015 to September 2017. Each soil column had a funnel at the bottom filled with quartz sand as a filter for solid particle removal. A plastic tube connected the funnel with a plastic bottle (4 L) that collected the deep-percolating water from the soil column. The first season with maize (spring-summer 2016) ran from October 2015 to March 2016, but samples were not taken because the soil columns were in a settling stage. After the maize was harvested (March 2016), dissolved N forms were measured from April 2016 until September 2016 (autumn-winter 1), from October 2016 until March 2017 (spring-summer 1) and from April 2017 until September 2017 (autumn-winter 2). Table 1 summarizes the physio-chemical properties of the soil columns studied. At field level, two horizons of the soil (0–42 cm and 42–50 cm) were sampled and a composite soil sample of five constituent samples per horizon was collected for chemical and physical characterisation. They were analysed as bulk simple sample following Chilean standard methods for soil chemical (Sadzawka et al., 2006) and physical analyses (Sandoval et al., 2012), including: soil pHwater (1:2.5, by potentiometry and pH meter), electrical conductivity (ECe, in soil extract with a conductivity meter), soil organic matter (SOM, by 400

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Table 1 Chemical and physical properties of the soil used in the column experiments. Horizon (depth)

Ap (0-42 cm) C (42-50 cm)

Soil propertiesa pHwater –

ECe dS m−1

SOM %

Db Mg m−3

AWC

Clay –

Silt

Sand

Textural class –

8.99 8.10

0.97 1.10

1.12 0.19

1.42 1.38

15 14

20.9 5.3

44.7 26.9

34.4 77.8

Loam Loamy sand

a ECe, extract electrical conductivity; SOM, soil organic matter; Db, bulk density; AWC, gravimetric available water content. Textural classes, according to Schoeneberger et al. (2012).

its emergence only the most vigorous and healthy plant was selected. Then maize crop management followed the common field crop practices, harvesting in March each year. Following harvest, maize stalks were shredded and incorporated into soil at 0–10 cm depth. The L. multiflorum (Lm) and T. repens (Tr) were sown at doses equivalent to 35 and 5 kg ha−1, respectively, in the soil columns. In the mixed Lm + Tr treatments, these were seed at a proportion of 30% Lm and 70% Tr according to the previous mentioned doses. In the maizecover crop treatments (see Table 2), the cover crops were harvested in September each year, before next maize planting; whereas in case of continuous cover crop treatments, these were cut in March and September.During the spring season, there were applied 150 kg N ha−1 using potassium nitrate at planting in treatments Lm-Lm150, Tr-Tr150, Zm–F400, Zm–Lm400, Zm–Tr400, and Zm–Lm + Tr400 and 250 kg N ha−1 using potassium nitrate after planting at V7 stage in Zm–F250, Zm–F400, Zm–Lm250, Zm–Lm400, Zm–Tr250, Zm–Tr400, Zm–Lm + Tr250 and Zm–Lm + Tr400.

Table 2 Description of the soil column experiment trials and treatments (in 4 replicates). Trial

Treatment/ Crop rotationa

Fertilisation rate (kg N ha−1)

1. Continuous fallow compared with continuous cover crop

F–F0 Lm–Lm0 Lm–Lm150 Tr–Tr0 Tr–Tr150 Zm–F250 Zm–F400 Zm–Lm250 Zm–Lm400 Zm–Tr250 Zm–Tr400 Zm–Lm + Tr250 Zm–Lm + Tr400

0 0 150 0 150 250 400 250 400 250 400 250 400

2. Maize-cover crop rotations

a F = fallow; Lm = Lolium multiflorum; Tr = Trifolium repens; Zm = Zea mays.

2.3. Sampling procedure and measurements

chromic acid wet oxidation), soil texture (Bouyoucos method), bulk density (Db, with cylinder) and soil water retention (-33 and −1500 kPa, with pressure devices) to estimate the available water capacity (AWC) of the soil.

During the study period, each column was irrigated weekly with 0.25–0.50 L (equivalent to 8–16 mm), depending on crop water demand and avoiding generation of deep percolation. However, deep water percolation was induced below the 50 cm soil column with an irrigation amount of 4 L (equivalent to 129 mm) applied to each column during both autumn-winter periods, to simulate an extreme rainfall event that may occurs in a Mediterranean Region (Mariani and Parisi, 2014), and during the spring-summer period, to simulate an excessive irrigation event with a furrow system that are usually applied by farmers in Mediterranean central Chile (Nájera et al., 2015). In total, there were nine excessive water application events with deep percolation: three in autumn-winter 2016, four in spring-summer 2016 and two in autumn-winter 2017. Assuming a water density of 1 Mg m−3 and measuring the water percolation mass (precision balance), the volume of percolated water in each column was weekly calculated. Crop evapotranspiration (ETc) was determined as the difference between the water losses measured as deep percolation and water inputs by irrigation assuming no changes (Allen et al., 1998). After each excess irrigation event, water samples of 100 mL collected below the soil column were chilled in coolers and delivered to the laboratory for further chemical analysis. The collected water samples were filtered through nitrate-free filters (0.45 μm) and the clear filtrate obtained was analysed for TDN, DIN (NO3-N and NH4-N) and DOC within 24 h. A chromotropic acid method (Hach kit, NitraVer® X Reagent Set, USA) was used to determine NO3-N and an ammonia-salicylate method (Hach kit, AmVerTM Nitrogen Ammonium Reagent, USA) to determine NH4-N concentrations in the collected water samples. Moreover, an UV–vis spectrophotometer (Hach DR5000, USA) was used to measure both DIN forms in the samples. The DOC and TDN concentrations were measured using a Shimadzu TOC-N analyser, and then DON was calculated as the difference between TDN measurements and DIN concentrations using colorimetric methods. The known column cross-section (0.031 m2) was used to express the DIN, DON and DOC loads in kg

2.2. Trials and treatments Table 2 summarizes the details of the soil columns, crop rotations, fertilizer treatments (4 replicates) and trials established to study the combined effects of inorganic N fertilisation and cover crops on maize yield and leaching of DIN, DON and DOC from the soil. The experiment established and studied four crop rotations namely, continuous fallow, continuous cover crop, maize followed by fallow, and maize followed by cover crop. These crop rotations were grouped in two trials to compare: 1) continuous fallow vs continuous cover crop; and 2) maizefallow vs maize-cover crop rotations. The experiment further included treatments with two cover crop types (i.e. L. multiflorum and T. repens) and different rates in inorganic N fertilizer applications (Table 2). For continuous cover crops, there were two rates, 0 and 150 kg N ha−1 of fertilizer applied. For grass crop, 150 kg N ha−1 represents over-fertilisation of 50% compared to the recommend N fertilization programs in Chile, but it is usually practised by farmers in the country. For maize, 250 kg N ha−1 corresponds to the optimum dose, whereas 400 kg N ha−1 represents over-fertilisation, which is usually practised by farmers in Central Chile (Nájera et al., 2015). In trial 2, the treatments with maize and fallow (Zm-F) had bare soil during autumnwinter (April 2016-September 2016 and April 2017-September 2017), while maize was cultivated during spring summer (October 2015March 2016 and October 2016-March 2017). In the treatments with maize and cover crops, cover crops were growing during autumn-winter (April 2016-September 2016 and April 2017-September 2017), while maize was cultivated during spring summer (October 2015-March 2016 and October 2016-March 2017). In the maize treatments two seeds per soil column were planted, and after 401

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ha−1, calculated as the volume of the percolated water multiplied with the solute concentration measured. Finally, samples of cover crop plant materials were collected from each soil column at the end of each period, to determine total dry mass at 70 °C (Sadzawka et al., 2004). 2.4. Statistical analysis After testing for normality of the data collected, statistical comparisons were made between the treatments studied (Table 2). To determine whether there were treatment effects on nitrogen and carbon leaching loads (DIN, DON and DOC) and ratios of soluble components (DON:DIN, DON:TDN and DOC:DON), Linear Mixed Models (LMMs) were performed and Fisher's least significant difference (LSD) test for comparisons according to:

Y = μ + Ri + τj + (Rτ ) ij + βik + Pi (k ) + εijk

Fig. 2. Effect of continuous cover crops and fertilization rates on overall soil deep percolation and crop evapotranspiration (ETc) from the soil columns over a period of April 2016 to September 2017 (see Table 2 for explanation of the treatment codes). Different letters on relevant bars indicate statistically significant differences between treatments (p < 0.05).

(1)

were: Y = outcome variable; μ= overall population mean; R = crop rotation x N fertilization effects; τ= time effect; Rτ= crop rotation x time effects; β= block effect; P = plot effect; ε= random errors. All statistical analyses were performed using Infostat Software.

solution under continuous fallow were measured significantly higher than in the continuous cover crop treatments (Table 3). Interestingly, the measured DIN concentration in the soil percolation solution for the continuous T. repens (Tr-Tr0 and Tr-Tr 150) treatment was on average two- to three-fold higher than for the continuous L. multiflorum (Lm-Lm0 and Lm-Lm150) treatment (Table 3). Moreover, L. multiflorum tended to display higher biomass than T. repens. This highlights that L. multiflorum grass species takes up more DIN from the soil solution, resulting in the lowest concentration of DIN leaching in the soil percolation solution, as also found by Korsaeth et al. (2003). The presence of the legume (T. repens) increased the DIN concentration in the soil solution through N-fixation, hence increased the availability and leaching of inorganic N in the soil. Shepherd et al. (2001) reported average values of DON equivalent to ∼8–10 kg ha−1 in the top 15 cm of several agricultural soils, with 75% of N in the soil solution. They also reported that, even on no manure applied soils, DON constituted about 8% of total N losses in soil drainage from eighteen soils (in a range of textures and management i.e. grass, arable and with manure applications) from UK. Jensen et al. (1997) reported a DON range of 8–30 kg ha−1 for sandy soils in Denmark, and McNeill et al. (1998) reported a DON range of 2–26 mg kg−1 during summer in soils under either continuous wheat or pasture in the Western Australia. In this study, the unfertilised cover crop treatments resulted the highest DON concentrations in soil percolation, followed by the fertilised cover crop treatments (Table 3). In addition, the treatments including L. multiflorum had higher DON concentrations values than the T. repens treatments (Tr-Tr0 and Tr-Tr150), with only non-detectable DON levels in F-F0 (Table 3). These results are within the range of values (0.7 to 3.1 mg L−1) reported in a review of DON leaching from pasture systems (van Kessel et al., 2009). However, our study and some European experiments (Sjöberg et al., 2003) on forest soils show a general trend for DON leaching to decrease with increasing N fertilizer input (Table 3). This is in contrast to observations in northern hardwood and pine forest under increased atmospheric deposition of N in USA (McDowell et al., 2004; Pregitzer et al., 2004). Our results suggest that inorganic N fertilisation favoured mineralisation of organic N forms in the soil. This is supported by the fact that the treatments with a cover crop and normal N fertilisation (Lm-Lm150 and Tr-Tr150) showed significantly lower DON concentrations than the corresponding treatments without N fertilisation (Lm-Lm0 and Tr-Tr0) (Table 3). The continuous fallow (F-F0) treatment received no organic N during the study period, which may explain its non-detectable DON concentrations (Table 3). The effects of cover crops were not observed that significant in terms of DOC concentrations measured in the soil percolation (Table 3). However, the treatment with T. repens without N fertilisation (Tr-Tr0) showed the highest DOC concentration, while the continuous fallow

3. Results and discussion 3.1. Trial 1: percolation and leaching of DIN, DON and DOC under continuous bare soil and continuous cover crop A comparison of the continuous cover crop treatments with the continuous bare soil treatment signifies effects of cover crops on soil water balance, soil percolation and leaching of nitrogen and carbon from the soil column experiments. In this comparison, continuous cover crops (Lm-Lm0, Lm-Lm150, Tr-Tr0 and Tr-Tr150) treatments resulted into the lower soil percolation than continuous fallow (F-F0) treatments during both autumn-winter periods of 2016 and 2017 (Fig. 1). Considering the overall period, continuous fallow yielded the highest deep percolation, denoting the absence of crop evapotranspiration expressed clearly by the cropping treatments (Fig. 2). Nitrogen mineralisation was the dominant process affecting DIN concentrations in the soil columns under continuous fallow (F-F0) treatment. In this case, leaching of DIN was the main source of N losses from the soil. The mean concentrations of DIN in soil percolation

Fig. 1. Effect of continuous cover crops on seasonal soil deep percolation measured from the soil columns (see Table 2 for explanation of treatment codes) during different periods. Different letters within periods indicate statistically significant differences between treatments in that period (p < 0.05). 402

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Table 3 Effect of maize-cover crops rotations and fertilisation rates on mean concentration (n = 9) of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) in the soil percolation solution and mean dry biomass of the cover crop (trial 1, n = 3; trial 2, n = 2) from the soil columns during the study period from April 2016 to September 2017 (See Table 2 for the treatment codes). Treatments

F-F0 Lm-Lm0 Lm-Lm150 Tr-Tr0 Tr-Tr150 Zm–F250 Zm–F400 Zm–Lm250 Zm–Lm400 Zm–Tr250 Zm–Tr400 Zm–Lm + Tr250 Zm–Lm + Tr400 a b

DIN1 mg L−1

DON1

Trial 1 21.95 ± 4.76 a 5.55 ± 2.86 d 7.50 ± 3.87 cd 14.06 ± 5.45 bc 17.20 ± 8.33 ab Trial 2 24.5 ± 9.13 bc 30.15 ± 7.13 ab 13.12 ± 5.21 d 25.75 ± 1.2 bc 27.8 ± 2.16 ab 36.65 ± 12.7 a 29.35 ± 3.21 ab 17.49 ± 7.13 cd

DOC1

Cover crop dry biomassa,b Mg ha−1

0.01 3.05 0.79 1.29 0.39

± ± ± ± ±

0.01 5.23 5.68 2.59 0.59

e a c b d

12.81 15.12 19.47 19.90 14.82

± ± ± ± ±

1.44 1.44 1.66 1.44 1.44

c bc ab a bc

– 18.70 21.09 16.90 15.16

± ± ± ±

4.96 2.57 3.32 1.43

ab a ab b

1.46 2.04 6.16 1.69 2.01 1.36 0.78 6.17

± ± ± ± ± ± ± ±

1.92 2.58 0.49 0.65 2.23 1.05 0.56 4.46

b b a b b b b a

15.65 14.32 21.28 19.99 16.91 15.54 20.02 30.46

± ± ± ± ± ± ± ±

3.83 3.83 3.83 4.30 3.83 3.83 3.83 3.83

b b ab b b b b a

– – 20.84 23.33 13.96 15.21 21.05 20.46

± ± ± ± ± ±

2.71 3.41 1.37 4.41 1.61 3.72

ab a b ab ab ab

Mean ± standard deviation. Different letters within columns indicate statistically significant differences between treatments (p < 0.05). Treatments Zm–Lm + Tr250 and Zm–Lm + Tr400 consider cover crop dry biomass of L. multiflorum and T. repens.

Therefore, L. multiflorum had higher DIN uptake and DON production in the soil columns than T. repens. As a result, in L. multiflorum the DON load increased in proportion to the greater litter production and microbial processing of soil organic N (Dijkstra et al., 2007). L. multiflorum resulted an average DON leaching load that was twice the DIN load (Table 4). Treatments F-F0 (continuous fallow) and Lm-Lm150 also had the highest DOC loads, with significant differences compared with the T. repens treatments. The treatments with L. multiflorum showed significantly higher DON:DIN and DON:TDN ratio and lower DOC:DON ratio than the other treatments. This is possibly because DON is linked to DOC load by sharing the same origin of organic matter (Wohlfart et al., 2012). We found that in bare soil the amount of DOC leaching was 16-fold higher than the DON leaching, which suggests that if an agricultural soil is fallow for a short time (< 2 years), the lack of organic matter turnover in soil and root exudates reduce the amount of DON leaching more rapidly than the DOC leaching.

treatment showed the lowest DOC concentration (Table 3). In the cover crop treatments, the DOC concentration in leachate did not show a clear tendency, probably due to the short study period (2 years). Similar to DON losses, the continuous fallow treatment had the lowest DOC concentrations, which may be related to the lack of organic matter addition during the study period. In addition, under continuous fallow (bare soil), no root exudates were produced, which may explain the lower DOC concentrations in continuous fallow treatment (Vinther et al., 2006). Table 4 summarizes the effects of different treatments on different solute leaching loads, integrating the effects on soil percolation and solute concentrations in soil percolation the soil columns. The continuous fallow treatment resulted the highest DIN loads, whereas the treatments with L. multiflorum (Lm-Lm0 and Lm-Lm150) had the lowest DIN loads (Table 4). This was the combined effect of lower soil percolation (Fig. 2) and lower DIN concentrations (Table 3) measured in the L. multiflorum treatments. The DIN load was significantly higher in T. repens than in L. multiflorum treatments. In contrast, the treatments with L. multiflorum showed the highest DON loads, whereas the fallow showed the lowest DON loads. This may be explained by L. multiflorum producing more above-ground biomass and root biomass than T. repens.

3.2. Trial 2: percolation and leaching of DIN, DON and DOC in maize-cover crop rotations Trial 2 assessed the effects of including cover crops as maize-cover

Table 4 Effect of maize-cover crops rotations and fertilization rates on mean loads and ratios of dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) (n = 9) in the soil percolation solution from the soil columns during the study period from April 2016 to September 2017 (See Table 2 for the treatment codes). Treatment

F-F0 Lm-Lm0 Lm-Lm150 Tr-Tr0 Tr-Tr150 Zm-F250 Zm-F400 Zm-Lm250 Zm-Lm400 Zm-Tr250 Zm-Tr400 Zm-Lm + Tr250 Zm-Lm + Tr400 a

DINa kg ha−1 Trial 1 237 ± 60 a 25 ± 9 c 37 ± 21 c 142 ± 63 b 140 ± 46 b Trial 2 163 ± 52 abc 207 ± 53 ab 82 ± 31 c 182 ± 52 abc 194 ± 87 ab 111 ± 62 bc 183 ± 80 abc 227 ± 123 a

DONa

DOCa

DON:DIN

DON:TDN

DOC:DON

6±5b 54 ± 28 a 62 ± 33 a 8±9b 9±6b

102 ± 8 a 88 ± 19 ab 107 ± 25 a 71 ± 8 b 77 ± 8 b

0.03 2.15 1.66 0.06 0.06

b a a b b

0.03 0.68 0.62 0.05 0.06

b a a b b

16 a 2c 2c 9b 9b

2±2c 6 ± 3 bc 16 ± 6 a 12 ± 5 ab 10 ± 12 abc 8 ± 5 abc 8 ± 5 abc 14 ± 2 ab

104 ± 10 bcd 88 ± 9 de 99 ± 10 cde 126 ± 12 ab 110 ± 10 bc 88 ± 9 ef 77 ± 9 f 153 ± 21 a

0.01 0.03 0.19 0.07 0.05 0.07 0.04 0.06

d c a b c b c bc

0.01 0.03 0.16 0.06 0.05 0.07 0.04 0.06

d c a bc c b c bc

44 a 16 ab 6c 10 b 11 b 11 b 10 b 11 b

Mean ± standard deviation. Different letters within columns indicate statistically significant differences between treatments (p < 0.05). 403

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to 360 kg N ha−1 in a wheat-maize crop rotation in China, the DIN concentration in leachate increased from 8 to 104 mg N L−1. In the present study, the maize cultivation with excessive N fertilization and T. repens as a cover crop (Zm–Tr400) resulted into the highest DIN concentration, while the maize cultivation with optimal N fertilization and L. multiflorum as a cover crop (Zm–Lm250) resulted into the lowest DIN concentration leaching from the soil columns (Table 3). As discussed above (in Trial 1), the L. multiflorum grass species takes up more DIN from the soil solution, resulting into the lowest concentration of DIN leaching in the soil percolation solution (Korsaeth et al., 2003). However, some treatments with L. multiflorum (Zm–Lm250 and Zm–Lm + Tr400) showed the highest DON concentrations in leachate (6 mg L−1) (Table 3), with significant differences compared with the other treatments. Huang et al. (2011) have reported DON concentrations ranging from 0.8 to 6.0 mg L−1 in leachate from winter wheat–summer maize double cropping systems in the North China. We did not find any effect of the N fertilisation dose applied to the maize crop in spring on DON concentrations, which suggest that most of the NO3-N was either uptake by the maize or lost by leaching. Other possible explanation is that microbial consumption of NO3- was not an important process in controlling soil NO3-N concentrations in the soil (Shi and Norton, 2000), then the DON concentration was not affected by the NO3-N microbial assimilation and decomposition. As in trial 1, in the winter cover crops the treatments with L. multiflorum had higher aboveground biomass than the T. repens (Table 3), which suggests that L. multiflorum had greater root biomass in the soil. This greater belowground biomass presumably favoured organic matter inputs to soil during autumn-winter, which resulted in higher DON concentrations in the soil solution and a higher risk of DON leaching. Another possible explanation is that much of the DON leached below the rooting zone in L. multiflorum treatments was recalcitrant and largely unavailable for plants and soil microorganisms. Delgado-Baquerizo et al. (2011) found that DON dominates over DIN in terms of concentrations on soils for most plant communities in winter, while the opposite occurs in summer in Mediterranean ecosystems. They also noted that plant canopies generating higher litter and organic matter accumulation beneath plants might explain the higher DON concentration in soil. Also, Dijkstra et al. (2007) found that DON leaching increased with greater litter production and microbial processing of soil organic N in grasslands in USA. Similar to DON losses, the crop rotation with maize, excess N fertilization and a cover crop mix L. multiflorum + T. repens (treatment Zm–Lm + Tr400) resulted into significantly highest DOC concentration in soil percolation from the soil columns (Table 3). In the other maizecover crop treatments, the DOC concentration in soil percolation did not show a clear tendency, probably due to the short study period (2 years). Table 4 summarizes the estimates of leaching loads of DIN, DON and DOC from different maize-cover crop rotations and fertilization treatments over the study period. The maize cultivation with excessive N fertilization and a cover crop mix L. multiflorum + T. repens treatment (i.e. Zm–Lm + Tr400) resulted into the highest DIN load (227 kg ha−1), whereas maize cultivation with optimal N fertilization and cover crop L. multiflorum treatment (i.e. Zm–Lm250) resulted into the lowest DIN load (82 kg ha−1) from the soil columns (Table 4). In treatment Zm–Lm250 the low DIN loads were related mainly to the combined effects of lowest soil deep percolation (Fig. 4), generated by higher evapotranspiration by the cover crop during autumn-winter, and optimum maize N fertilisation during spring-summer leading to lowest DIN concentrations in the soil percolation (Table 3). In contrast, treatment Zm–Lm250 showed the highest DON loads because it had the highest DON concentrations in the soil percolation. The treatment Zm–Lm + Tr400 had the highest DOC loads (Table 4), due to having the highest DOC concentrations in the soil percolation (Table 3). Overall, the Zm–Lm250 treatment showed significant higher DON:DIN and DON:TDN ratio and lower DOC:DON ratio than the other

Fig. 3. Effect of maize-cover crop rotations and fertilization rates on seasonal soil deep percolation measured from the soil columns (see Table 2 for explanation of the treatment codes). Different letters on bars within a period indicate statistically significant differences between treatments in that period (p < 0.05).

Fig. 4. Effect of maize-cover crop rotations and fertilization rates on overall soil deep percolation and crop evapotranspiration (ETc) from the soil columns over a period of April 2016 to September 2017 (see Table 2 for explanation of the treatment codes). Different letters on relevant bars indicate statistically significant differences between treatments (p < 0.05).

crop rotations and fertilization rates on soil water balance, soil percolation and leaching of N and C from the soil column experiments. During the autumn-winter seasons of 2016 and 2017, the maize-cover crop treatments that included L. multiflorum (Zm–Lm250, Zm–Lm400, Zm–Lm + Tr250 and Zm–Lm + Tr400) resulted into lower soil percolation than the treatments with fallow and T. repens (Fig. 3). However, during the spring-summer 2016, when a maize crop was present in all treatments, there was no clear tendency in soil percolation except that the lowest percolation occurred in the treatment that received excess N (Zm–F400) (Fig. 3), which was associate with a high maize biomass growth (data not shown). Overall, the treatments that included L. multiflorum (Zm–Lm250, Zm–Lm + Tr250 and Zm–Lm + Tr400) resulted into lower soil percolation than the treatments with fallow and T. repens (Zm–Tr250 and Zm–Tr400) over the study period from April 2016 to September 2017 (Fig. 4). Considering the amount of water applied and soil, percolation measured, our result suggest that maize cultivation treatments with L. multiflorum showed higher evapotranspiration than the fallow and T. repens treatments due to higher biomass growth during autumn-winter period (Fig. 4). Inclusion of cover crops as maize-cover crop rotations and fertilization rates had significant effect on mean concentrations of DIN, DON and DOC during the study period (Table 3). As expected, excessive N fertilizer application (400 kg N ha−1) increased the DIN concentrations in leachate, but the increase was only significant on Zm–Tr400 (Table 3). Huang et al. (2011) found that when fertilisation was increased from 0

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cover crop (L. multiflorum) generated the lowest amount of TDN leaching from the soil columns. We recommend this to be further studied in field conditions as a best management practice for reducing the risk of diffuse pollution of surface water bodies and groundwater from maize cultivation in Mediterranean Central Chile.

treatments, whereas the rotations with fallow (Zm–F250 and Zm–F400) showed the lowest DON:DIN and DON:TDN ratio and the highest DOC:DON ratio. Vinther et al. (2006) noted that fresh, younger organic matter produced during recent years contributes more to the dissolved pools of organic C and N than the old humus fraction of the soil, which may explain why cover crop treatments showed higher DON leaching than bare soil. Similar to trial 1, in trial 2 we found that including a fallow period and the associated lack of organic additions to soil decreased the amount of DON leaching more rapidly than the DOC leaching in soil percolation from the soil columns. It is clear that this study evaluated the worst possible scenarios favouring DIN leaching, i.e. a coarse-textured soil, extremely high-intensity precipitation or irrigation events (with application of 129 mm) and excess N applied (400 kg N ha1). However, the results supports the earlier findings of a potential risk of DIN leaching due to excessive N fertilisation for maize cultivation, which can result in substantial N leaching losses under high-intensity precipitation and excessive irrigation in Central Chile (Salazar et al., 2014; Corradini et al., 2015; Salazar et al., 2017). This could be addressed by inclusion of a cover crop L. multiflorum and optimal N fertilisation treatment (i.e. Zm–Lm250) for maize cultivation on coarse textured soils in Central Chile. The inclusion of cover crops, however, could lead to an increase in leaching of DON and DOC in the soil percolation (Table 4). If leached DON moves through the soil and is mineralised below the rooting depth it will also contribute to the TDN load, but if mineralised within the rooting depth it may eventually contribute to the soil N supply to the crop. The leached DOC could be an important energy source for denitrifying bacteria in deeper soil layers and thereby reduce leaching of dissolved N forms to groundwaters (Vinther et al., 2006). Therefore, further information is required for effects of cover crops on the mobility and degradability of DON and DOC pools and their fate in the deeper soil layers under maize-cover crop rotations in Central Chile. While much research has been devoted to quantifying the amount and flux of dissolved organic nutrients in freshwater and marine systems, less has been performed on soils. The findings of this study and further research in leaching and fate of DON as an N sink in agricultural production systems will be crucial to improve our farm management and cultivation practices to help reduce their environmental impacts on receiving waters across agricultural landscapes.

Acknowledgements The authors thank the Department of Soil and Engineering at the University of Chile for supporting this study. This research was partially funded by FONDECYT Regular 2015 grant no. 1150572. References Abaas, E., Hill, P.W., Roberts, P., Murphy, D.V., Jones, D.L., 2012. Microbial activity differentially regulates the vertical mobility of nitrogen compounds in soil. Soil Biol. Biochem. 53, 120–123. Allen, R.G., Pereira, L.S., Raes, D., Smith, M., 1998. Crop Evapotranspiration—Guidelines for Computing Crop Water Requirements—FAO Irrigation and Drainage Paper 56. FAO—Food and Agriculture Organization of the United Nations, Rome, Italy 293 pp. Bartley, R., Speirs, W.J., Ellis, T.W., Waters, D.K., 2012. A review of sediment and nutrient concentration data from Australia for use in catchment water quality models. Mar. Pollut. Bull. 65, 101–116. Berenguer, P., Santiveri, F., Boixadera, J., Lloveras, J., 2009. Nitrogen fertilisation of irrigated maize under Mediterranean conditions. Eur. J. Agron. 30, 163–171. Campbell, J.L., Socci, A.M., Templer, P.H., 2014. Increased nitrogen leaching following soil freezing is due to decreased root uptake in a northern hardwood forest. Glob. Change Biol. Bioenergy. https://doi.org/10.1111/gcb.12532. Casanova, M., Seguel, O., Salazar, O., Luzio, W., 2013. Soils of Chile. Soils of the World Soils Serie. Springer Science+Business Media, Germany. Christou, M., Avramides, E.J., Jones, D.L., 2006. Dissolved organic nitrogen dynamics in a Mediterranean vineyard soil. Soil Biol. Biochem. 38, 2265–2277. CIREN, 1996. Estudio Agrológico. descripción de suelos materiales y símbolos: Región Metropolitana. (Soil survey. Soil descriptions, materials and symbols: Región Metropolitana). Centro de Información de Recursos Naturales Publicación no. 115. (in Spanish). Corradini, F., Nájera, F., Casanova, M., Tapia, Y., Singh, R., Salazar, O., 2015. Effects of maize cultivation on nitrogen and phosphorus loadings to drainage channels in central Chile. Environ. Monit. Assess. 187, 697. Delgado-Baquerizo, M., Covelo, F., Gallardo, A., 2011. Dissolved organic nitrogen in Mediterranean ecosystems. Pedosphere 21, 309–318. Dijkstra, F.A., West, J.B., Hobbie, S.E., Reich, P.R., Trost, J., 2007. Plant diversity, CO2, and N influence inorganic and organic N leaching in grasslands. Ecology 88, 490–500. Evans, C.D., Monteith, D.T., Cooper, D.M., 2005. Long-term increases in surface water dissolved organic carbon: observations, possible causes and environmental impacts. Environ. Pollut. 137, 55–71. Evans, D.M., Schoenholtz, S.H., Wigington Jr, P.J., Griffith, S.M., Floyd, W.C., 2014. Spatial and temporal patterns of dissolved nitrogen and phosphorus in surface waters of a multi-land use basin. Environ. Monit. Assess. 186, 873–887. Fuentes, I., Casanova, M., Seguel, O., Nájera, F., Salazar, O., 2014. Morpho-physical pedotransfer functions for groundwater pollution by nitrate leaching in Central Chile. Chil. J. Agric. Res. 74, 340–348. Gabriel, J.L., Quemada, M., 2011. Replacing bare fallow with cover crops in a maize cropping system: yield, N uptake and fertiliser fate. Eur. J. Agron. 34, 133–143. Gabriel, J.L., Muñoz-Carpena, R., Quemada, M., 2012. The role of cover crops in irrigated systems: water balance, nitrate leaching and soil mineral nitrogen accumulation. Agric. Ecosyst. Environ. 115, 50–61. Galloway, J.N., Townsend, A.R., Erisman, J.W., Bekunda, M., Cai, Z., Freney, J.R., Martinelli, L.A., Seitzinger, S.P., Sutton, M.A., 2008. Transformation of the nitrogen cycle: recent trends, questions, and potential solutions. Science 320, 889–892. Gentry, L.E., Snapp, S.S., Price, R.F., Gentry, L.F., 2013. Apparent red clover nitrogen credit to corn: evaluating cover crop introduction. Agron. J. 105, 1658–1664. Hoeft, I., Keuter, A., Quiñones, C.M., Schmidt-Walter, P., Veldkamp, E., Corre, M.D., 2014. Nitrogen retention efficiency and nitrogen losses of a managed and phytodiverse temperate grassland. Basic Appl. Ecol. 15, 207–218. Huang, M., Liang, T., Ou-Yang, Z., Wang, L., Zhang, C., Zhou, C., 2011. Leaching losses of nitrate nitrogen and dissolved organic nitrogen from a yearly two crops system, wheat-maize, under monsoon situations. Nutr. Cycl. Agroecosyst. 91, 77–89. Isidoro, D., Quílez, D., Aragüés, R., 2006. Environmental impact of irrigation in La Violada district (Spain). J. Environ. Qual. 35, 776–785. Jensen, L.S., Mueller, T., Magid, J., Nielsen, N.E., 1997. Temporal variations of C and N mineralization, microbial biomass and extractable organic pools in soil after oilseed rape straw incorporation in the field. Soil Biol. Biochem. 29, 1043–1055. Jones, D.L., Shannon, D., Murphy, D.V., Farrar, J., 2004. Role of dissolved organic nitrogen (DON) in soil N cycling in grassland soils. Soil Biol. Biochem. 36, 749–756. Kiikkilä, O., Smolander, A., Kitunen, V., 2013. Degradability, molecular weight and adsorption properties of dissolved organic carbon and nitrogen leached from different types of decomposing litter. Plant Soil 373, 787–798. Korsaeth, A., Bakken, L.R., Riley, H., 2003. Nitrogen dynamics of grass as affected by N input regimes, soil texture and climate: lysimeter measurements and simulations.

4. Conclusions The study clearly shows that inclusion of cover crops in maize cultivation had significant effects on cycling and losses of DIN, DON and DOC from a coarse-textured soil column experiment. Using the soil column experiments, we found that inclusion of a grass cover crop (L. multiflorum) and optimal N fertilisation (250 kg N ha−1) treatment resulted into lower DIN losses from maize cultivation on a coarse textured soil. In trial 1, we found that inclusion of a continuous grass cover crop (L. multiflorum) resulted into the DON loading as the dominant form of nitrogen leaching (on average 65% of TDN loading), while inclusion of a continuous legume cover crop (T. repens) or fallow, the DON leaching load was an order of magnitude lower than the DIN load from the soil columns. We also found that when an agricultural soil is under fallow, the lack of turnover of organic matter in soil and root exudates reduces DON leaching more rapidly than DOC leaching, giving lower DON:DIN and DON:TDN load ratios, but higher DOC:DON load ratio, than cover cropping in the short time scale as studied here (2 years). In trial 2, we found that a crop rotation with maize, excessive N fertilization (400 kg N ha−1) and inclusion of a legume cover crop (T. repens) showed the highest DIN leaching load, whereas a crop rotation with maize, optimal N fertilisation (250 kg N ha−1) and a grass cover crop (L. multiflorum) had the lowest DIN loads from the soil columns. However, the latter treatment had significantly higher DON loads (16% of TDN loads) than all other maize-fallow treatments. Overall, the rotation with maize, optimal N fertilisation (250 kg N ha−1) and a grass 405

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Sadzawka, A., Grez, R., Carrasco, M.A., Mora, M., 2006. Métodos Recomendados Para Suelos Chilenos (Methods Recommended for Chilean Soils). Comisión Nacional de Normalización y Acreditación. Sociedad Chilena de la Ciencia del Suelo, Santiago, Chile (in Spanish). Salazar, O., Wesström, I., Joel, A., Youssef, M.A., 2013. Application of an integrated framework for estimating nitrate loads from a coastal watershed in south-east Sweden. Agric. Water Manage. 129, 56–68. Salazar, O., Vargas, J., Nájera, F., Seguel, O., Casanova, M., 2014. Monitoring ofnitrate leaching during flush flooding events in a coarse-textured floodplainsoil. Agric. Water Manage. 146, 218–227. Salazar, O., Nájera, F., Tapia, W., Casanova, M., 2017. Evaluation of the DAISY model for predicting nitrogen leaching in coarse-textured soils cropped with maize in the Mediterranean zone of Chile. Agric. Water Manage. 182, 77–86. Salmerón, M., Isla, R., Cavero, J., 2011. Effect of winter cover crop species and planting methods on maize yield and N availability under irrigated Mediterranean conditions. Field Crop. Res. 123, 89–99. Sandoval, M., Dörner, J., Seguel, O., Cuevas, J., Rivera, D., 2012. Métodos de análisis físicos de suelos (Methods of physical soil analyses). Universidad de Concepción. Publicaciones Departamento de Suelos y Recursos Naturales N° 5, Chillán, Chile. Schoeneberger, P.J., Wysocki, D.A., Benham, E.C., 2012. Field Book for Describing and Sampling Soils, Version 3.0. Natural Resources Conservation Service, National Soil Survey Center, Lincoln, NE, USA. Scott, E.E., Rothstein, D.E., 2014. The dynamic exchange of dissolved organic matter percolating through six diverse soils. Soil Biol. Biochem. 69, 83–92. Shepherd, M., Bhogal, A., Barret, G., Dyers, C., 2001. Dissolved organic nitrogen in agricultural soils: effects of sample preparation on measured values. Commun. Soil Sci. Plant Anal. 32 (9-10), 1523–1542. Shi, W., Norton, J.M., 2000. Microbial control of nitrate concentrations in an agricultural soil treated with dairy waste compost or ammonium fertilizer. Soil Biol. Biochem. 32, 1453–1457. Simeonova, T., Stoicheva, D., Koleva, V., Sokołowska, Z., Hajnos, M., 2017. Effect of longterm fertilizer application in maize crop growing on chemical element leaching in Fluvisol. Int. Agrophys. 31, 243–249. Sjöberg, G., Bergkvist, B., Berggren, D., Nilsson, S.I., 2003. Long term N addition effects on the C mineralisation and DOC production in mor humus under spruce. Soil Biol. Biochem. 35, 1305–1315. van Kessel, C., Clough, T., van Groenigen, J.W., 2009. Dissolved organic nitrogen: an overlooked pathway of nitrogen loss from agricultural systems? J. Environ. Qual. 38, 393–401. Vinther, F.P., Hansen, E.M., Eriksen, J., 2006. Leaching of soil organic carbon and nitrogen in sandy soils after cultivating grass-clover swards. Biol. Fertil. Soils 43, 12–19. Vitousek, P.M., Naylor, R., Crews, T., David, M.B., Drinkwater, L.E., Holland, E., Johnes, P.J., Katzenberger, J., Martinelli, L.A., Matson, P.A., Nziguheba, G., Ojima, D., Palm, C.A., Robertson, G.P., Sanchez, P.A., Townsend, A.R., Zhang, F.S., 2009. Nutrient imbalances in agricultural development. Science 324, 1519–1520. Wagger, M.G., Cabrera, M.L., Ranells, N.N., 1998. Nitrogen and carbon cycling in relation to cover crop residue quality. J. Soil Water Conserv. 53, 214–218. Wohlfart, T., Exbrayat, J.-F., Schelde, K., Christen, B., Dalgaard, T., Frede, H.-G., Breuer, L., 2012. Spatial distribution of soils determines export of nitrogen and dissolved organic carbon from an intensively managed agricultural landscape. Biogeosciences 9, 4513–4525. Worrall, F., Harriman, R., Evans, C.D., Watts, C.D., Adamson, J., Neal, C., Tipping, E., Burt, T., Grieve, I., Monteith, D., Naden, P.S., Nisbet, T., Reynolds, B., Stevens, P., 2004. Trends in dissolved organic carbon in UK rivers and lakes. Biogeochemistry 70, 369–402.

Nutr. Cycl. Agroecosyst. 66, 181–199. Lal, R., 2013. Intensive agriculture and the soil carbon pool. J. Crop Improv. 27, 735–751. Macdonald, B.C.T., Ringrose-Voase, A.J., Nadelko, A.J., Farrell, M., Tuomi, S., Nachimuthu, G., 2016. Dissolved organic nitrogen contributes significantly to leaching from furrow-irrigated cotton–wheat–maize rotations. Soil Res. 55, 70–77. Manninen, N., Soinne, H., Lemola, R., Hoikkala, L., Turtola, E., 2018. Effects of agricultural land use on dissolved organic carbon and nitrogen in surface runoff and subsurface drainage. Sci. Total Environ. 618, 1519–1528. Mariani, L., Parisi, S.G., 2014. Extreme rainfalls in the Mediterranean Area. In: In: Diodato, N., Bellocchi, G. (Eds.), Storminess and Environmental Change, Advances in Natural and Technological Hazards Research, vol 39. Springer Science+Business Media, Dordrecht, pp. 17–37. Mattsson, T., Kortelainen, P., Räike, A., 2005. Export of DOM from boreal catchments: impacts of land use cover and climate. Biogeochemistry 76, 373–394. Mattsson, T., Kortelainen, P., Laubel, A., Evans, D., Pujo-Pay, M., Räike, A., Conan, P., 2009. Export of dissolved organic matter in relation to land use along a European climatic gradient. Sci. Total Environ. 407, 1967–1976. McDowell, W.H., Magill, A.H., Aitkenhead-Peterson, J.A., et al., 2004. Effects of chronic nitrogen amendment on dissolved organic matter and inorganic nitrogen in soil solution. For. Ecol. Manage. 196, 29–41. McGovern, S.T., Evans, C.D., Dennis, P., Walmsley, C.A., Turner, A., McDonald, M.A., 2014. Increased inorganic nitrogen leaching from a mountain grassland ecosystem following grazing removal: a hangover of past intensive land-use? Biogeochemistry 119, 125–138. McNeill, A.M., Sparling, G.P., Murphy, D.V., Braunberger, P., Fillery, I., 1998. Changes in extractable and microbial C, N and P in a Western Australian wheatbelt soil following simulated summer rainfall. Austr. J. Soil Res. 36, 841–854. Murphy, D.V., Macdonald, A.J., Stockdale, E.A., Goulding, K.W.T., Fortune, S., Gaunt, J.L., Poulton, P.R., Wakefield, J.A., Webster, C.P., Wilmer, W.S., 2000. Soluble organic nitrogen in agricultural soils. Biol. Fertil. Soils 30, 374–387. Nájera, F., Tapia, Y., Baginsky, C., Figueroa, V., Cabeza, R., Salazar, O., 2015. Evaluation of soil fertility and fertilisation practices for irrigated maize (Zea mays L.) Under Mediterranean conditions in central Chile. J. Soil Sci. Plant Nutr. 15, 84–97. Näsholm, T., Kielland, K., Ganeteg, U., 2009. Uptake of organic nitrogen by plants. New Phytol. 182, 31–48. Necpalova, M., Fenton, O., Casey, I., Humphreys, J., 2012. N leaching to groundwater from dairy production involving grazing over the winter on a clay-loam soil. Sci. Total Environ. 432, 159–172. Neff, J.C., Chapin III, F.S., Vitousek, P.M., 2003. Breaks in the cycle: dissolved organic nitrogen in terrestrial ecosystems. Front. Ecol. Environ. 1, 205–211. Pregitzer, K.S., Zak, D.R., Burton, A.J., Ashby, J.A., MacDonald, N.W., 2004. Chronic nitrate additions dramatically increase the export of carbon and nitrogen from northern hardwood ecosystems. Biogeochemistry 68, 179–197. Rasmussen, J., Gjettermann, B., Eriksen, J., Jensen, E.S., Høgh-Jensen, H., 2008. Fate of 15N and 14C from labelled plant material: recovery in perennial ryegrass-clover mixtures and in pore water of the sward. Soil Biol. Biochem. 40, 3031–3039. Riaz, M., Mian, I.A., Bhatti, A., Cresser, M.S., 2012. An exploration of how litter controls drainage water DIN, DON and DOC dynamics in freely draining acid grassland soils. Biogeochemistry 107, 165–185. Ros, G.H., Hoffland, E., van Kessel, C., Temminghoff, E.J., 2009. Extractable and dissolved soil organic nitrogen–A quantitative assessment. Soil Biol. Biochem. 41, 1029–1039. Sadzawka, A., Grez, R., Carrasco, M.A., Mora, M., 2004. Métodos Recomendados Para Tejidos Vegetales (Recommended Methods for Vegetal Tissues). Comisión Nacional de Normalización y Acreditación, Sociedad Chilena de la Ciencia del Suelo, Santiago, Chile (in Spanish).

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