Influence of long-term corn–soybean crop sequences on soil ecology as indicated by the nematode community

Applied Soil Ecology 100 (2016) 172–185

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Influence of long-term corn–soybean crop sequences on soil ecology as indicated by the nematode community Zane J. Grabau* , Senyu Chen Southern Research and Outreach Center, Department of Plant Pathology, University of Minnesota, 35838 120th Street Waseca, MN 56093, United States

A R T I C L E I N F O

A B S T R A C T

Article history: Received 15 September 2015 Received in revised form 29 December 2015 Accepted 31 December 2015 Available online xxx

In the Midwestern United States, corn–soybean rotation is an essential agricultural practice, but relatively little is known about the impact of different corn–soybean cropping sequences on soil ecology. A long-term research site in Waseca, Minnesota was established in 1982 to study corn–soybean rotation. At the site, various corn–soybean crop sequences can be compared each year including corn and soybean in 1 to 5 years of monoculture and continuous monoculture of each crop. Additionally, granular nematicides (terbufos or aldicarb) have been applied to half of each plot since 2010 to minimize nematode populations, particularly plant-parasitic nematodes, across crop sequences. The nematode community, a sensitive indicator of changes in soil ecology, was assessed at this site to determine the impact of corn–soybean crop sequences and nematicide application on the soil ecosystem. Nematicide application was effective against target nematodes, herbivores, but also impacted non-target nematodes and thus soil ecology. Nematicide application decreased fungivore and bacterivore populations, diversity, and maturity; but significantly increased enrichment compared to no nematicide application. The nematode community and thus soil ecology was significantly different in corn compared to soybean cropping systems and changed most during initial years after switching crops. Cropping systems in corn supported significantly greater fungivore populations, fungal decomposition pathways, more diversity, and a more mature ecosystem compared to soybean systems. Soybean systems supported significantly greater bacterivore populations and a more disturbed, enriched ecosystem. These differences between corn and soybean systems demonstrate that each crop has a distinct impact on the soil ecosystem. ã 2016 Elsevier B.V. All rights reserved.

Keywords: Nematode community Corn Soybean Crop rotation Soil ecology

1. Introduction Crop rotation is a common practice in agricultural systems to maintain crop productivity particularly for corn (Zea mays L.) and soybean (Glycine max L. Merr.). In the United States, corn–soybean rotation is among the most important agronomic systems and is a major feature of the landscape. In 2014, 37 and 34.3 million hectares of corn and soybean respectively were planted in the United States which is 53.5% of total area planted to principal crops (NASS-USDA, 2014) or 4% of total land area (Nickerson et al., December 2011). Most research on corn–soybean rotation has focused on agronomic factors such as crop yield (Crookston et al., 1991; Crookston and Kurle, 1989; Howard et al., 1998; Porter et al., 1997; Wilhelm and Wortmann, 2004), soil nutrients (Meese et al., 1991; Omay et al., 1998; Peterson and Varvel, 1989), pathogen populations (Howard et al., 1998; Porter et al., 2001; Whiting and Crookston, 1993), other soil properties (Copeland et al., 1993;

* Corresponding author. Fax: +1 507 835 3622. E-mail address: [email protected] (Z.J. Grabau). http://dx.doi.org/10.1016/j.apsoil.2015.12.016 0929-1393/ ã 2016 Elsevier B.V. All rights reserved.

Meese et al., 1991), or plant physiology (Copeland and Crookston, 1992; Nickel et al., 1995; Pikul et al., 2012). Less is known about the impact of different cropping systems on soil biology and ecology. Since corn–soybean systems are so common, a better understanding of this system would provide a better understanding of our landscape. Additionally, a better understanding of soil ecology under different cropping systems may give insight into mechanisms behind agronomic benefits of crop rotation and help determine optimal practices for maintaining productive soil. The nematode community is a dynamic indicator of soil ecology because it spans a wide range of trophic groups and ecological niches, and is sensitive to changes in the environment (Bongers, 1990; Ferris et al., 2001; Fiscus and Neher, 2002). The nematode community has been used as a tool for assessing various management practices in agricultural systems including tillage (Okada and Harada, 2007; Sanchez-Moreno et al., 2006; Villenave et al., 2009), fertilizer application (Hu and Cao, 2008; Leroy et al., 2009; Liang et al., 2009; Villenave et al., 2010), and organic management practices (Dong et al., 2008; Overstreet et al.,

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corn–soybean crop sequence treatments have been maintained continuously since 1982. The three sequence types including 14 crop sequence treatments (Table 1) were: (i) five years corn followed by 5 years soybean with each phase grown each year such that both crops have treatments in years 1, 2, 3, 4, and 5 of monoculture every year; (ii) continuous monoculture of each crop; (iii) annual rotation between two cultivars – but crop monoculture – of each crop. Since 1995, sequence type (iii) has been singlecultivar monoculture of corn or soybean. Beginning in 2010, treatments in sequences (i) and (ii) were corn cultivars with Bacillus thuringiensis trait (Bt) or SCN-susceptible soybean cultivars while sequence (iii) was corn cultivars without Bt trait or SCNresistant soybean. From 2010 onward, half of each plot was treated with in-furrow, granular nematicide to create a split-plot experiment arrangement with subplots 4.57 m wide by 7.62 m containing 6 crop rows. In 2010 and 2011, terbufos nematicide (Counter 20G, AMVAC Chemical Corporation) was applied in-furrow at planting at 2.44 kg a.i. ha1. In 2012–2014, aldicarb nematicide (Bolster 15G, AMVAC Chemical Corporation) was applied in-furrow at planting at 2.94 kg a.i. ha1. For both nematicides, these rates, which were approximately double the label rate, were used to achieve maximum nematode control for completing the research objectives. Both crop sequence and nematicide factors were randomized complete block designs with 4 replicates within the split-plot arrangement.

2010). Some studies have examined the influence of different cropping systems on the nematode community (Briar et al., 2012; Carter et al., 2009; Djigal et al., 2012; Govaerts et al., 2006; Osler et al., 2000; Rahman et al., 2007), but none have focused on corn– soybean rotations in a temperate climate. Examining distinct cropping sequences over an extended time period may reveal trends that are not apparent over a shorter time period and reflects the long time periods that agricultural fields remain in production. In 1982, a long-term field study involving various corn and soybean crop sequences was initiated in Waseca, Minnesota to examine agronomic aspects of corn–soybean rotation when soil nutrients are not limiting. This site is a unique opportunity to examine the influence of corn–soybean crop rotations on soil ecology and the nematode community. To help determine the role of nematodes in agronomic aspects of crop rotation, crop sequences with and without nematicide application have been maintained at the site since 2010. In particular, soybean cyst nematode (SCN, Heterodera glycines) is the major pathogen of soybean in this area, causing yield losses of 30% or more in some cases (Chen et al., 2001). However, environmental impacts of pesticide application are increasingly under scrutiny with many nematicides no longer approved for use (Rich et al., 2004). Additionally, nematicides can impact both target nematodes that damage plants and non-target nematodes (Chelinho et al., 2011; De Bruin and Pedersen, 2008; Sanchez-Moreno et al., 2010) that provide beneficial services in the soil ecosystem (Ferris et al., 2012) making it important to understand the full impact of these applications. Nematicide can also be a tool to understand the role of nematodes in the impacts of crop rotation on other agronomic factors. The objective of this study was to assess impact of nematicide application and long-term crop rotations on soil ecology based on the nematode community.

2.2. Soil sampling and nematode assessment Soil samples for nematode community analysis were collected in 2013 and 2014 at three times during each year: Spring (within 2 days before planting), midseason (47–64 days after planting [DAP]), and fall (at harvest) from all subplots. From each subplot, 20 soil cores were taken in the two central rows (within 10 cm of plant rows) to a depth of 20 cm. Soil samples were homogenized by passing through a metal screen with 4 mm apertures before further processing. In 2013, soil samples were collected on 3 June (day of planting, but before seeds were planted), 6 August (64 DAP), and 8 October (127 DAP). In 2014, soil samples were collected on 19 May (2 days before planting), 7 July (47 DAP), and 9 October (139 DAP). 128 soil samples were analyzed each season for a total of 768 soil samples analyzed during the study.

2. Materials and methods 2.1. Experimental design The study was conducted in a Nicollet clay loam (fine-loamy, mixed, mesic Aquic Hapludoll; pH 6.5; 5.5% organic matter) at the Southern Research and Outreach Center in Waseca, Minnesota (44
040 N, 93
330 W) at a field site where plots of various Table 1 Corn (C) and soybean (S) cropping sequence treatmentsa in Waseca, MN. Treatments

10-year rotation 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Crop sequence by year 2005

2006

2007

2008

2009

2010

2011

2012

2013

2014

C4 C3 C2 C1 S5 S4 S3 S2 S1 C5

C5 C4 C3 C2 C1 S5 S4 S3 S2 S1

S1 C5 C4 C3 C2 C1 S5 S4 S3 S2

S2 S1 C5 C4 C3 C2 C1 S5 S4 S3

S3 S2 S1 C5 C4 C3 C2 C1 S5 S4

S4 S3 S2 S1 C5 C4 C3 C2 C1 S5

S5 S4 S3 S2 S1 C5 C4 C3 C2 C1

C1 S5 S4 S3 S2 S1 C5 C4 C3 C2

C2 C1 S5 S4 S3 S2 S1 C5 C4 C3

C3 C2 C1 S5 S4 S3 S2 S1 C5 C4

Cc Ss

Cc Ss

Cc Ss

Cc Ss

Cc Ss

Cc Ss

Cc Ss

Cc Ss

Cc Ss

Cn Sr

Cn Sr

Cn Sr

Cn Sr

Cn Sr

Continuous Monoculture 11. Cc 12. Ss

Continuous; non-Bt corn and SCN-resistance soybean post-2010, alternating cultivars pre-1995 13. Cc Cc Cc Cc Cc 14. Ss Ss Ss Ss Ss

a Cc and Cn are continuous corn with non-Bt and Bt cultivars since 2010 respectively; C1 through C5 are 1st- to 5th-year corn after 5 years of soybean; S1 through S5 are 1stto 5th-year soybean following 5 years of corn; Ss and Sr are continuous soybean with SCN-susceptible and resistant cultivars respectively since 2010. All soybeans, except Sr, were susceptible to SCN.

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Z.J. Grabau, S. Chen / Applied Soil Ecology 100 (2016) 172–185 Table 2 Effects of crop sequence and nematicide on nematode populations and nematode community indices for 2013–2014 combined data.

Vermiform nematode population densities were determined for all soil samples collected at spring, midseason, and fall in 2013 and 2014. Vermiform nematodes from each subplot were extracted from a 100 cm3 homogenized soil subsample using a modified sucrose floatation and centrifugation method (Jenkins, 1964). Nematode abundance was calculated based on a subsample of at least 10% of extracted nematodes. A subsample of at least 100 nematodes from each subplot was identified morphologically to genus using a light microscope and soil population density for vermiform stages of each genus was calculated. Based on this information, abundances (nematodes 100 cm3 soil) of total nematodes, herbivores, bacterivores, fungivores, omnivores, and predators were calculated (Yeates et al., 1993). Selected nematode community indices were also calculated including ShannonWeaver diversity index, maturity index (MI), MI25, plant-parasite index (PPI), enrichment index (EI), basal index (BI), structure index (SI), channel index (CI), and the ratio of fungivores and bacterivores to herbivores (FBPP). The Shannon–Weaver diversity index is a measure of the diversity of nematode genera in a location (Neher et al., 1995). The maturity index and MI25 measure disturbance based on average colonizer-persister scale values, classified by ecological life strategies, of nematodes in the soil (Bongers, 1990). While the maturity index includes all free-living nematodes, MI25 excludes nematodes with colonizer-persister value 1— extreme colonizers (Bongers, 1990). The plant-parasite index is a similar measure to MI and MI25, but includes only herbivores (Bongers, 1990). The enrichment, basal, and structure indices measure the enrichment, basal, and structure conditions respectively of the food web based on indicator nematode guilds (Ferris et al., 2001). Similarly, the channel index measures whether fungal or bacterial decomposition pathways are more predominant in a system (Ferris et al., 2001). FBPP is the ratio of fungivores and bacterivores to herbivores and helps indicate the relative positive or negative impact of nematodes (Wasilewska, 1989).

at 78 kg ha1 in the form of triple super phosphate and K at 39 kg ha1 in the form of potash.

2.3. Site management

2.4. Statistical analysis

Corn and soybean were planted, with concurrent nematicide applications to appropriate subplots, on 3 June 2013; and 21 May 2014. Corn cultivars planted were ‘De Kalb 50–66’ (Bt-trait corn sequences) and ‘DeKalb 50-67’ (Bt-free corn sequences). Soybean cultivars planted were ‘Pioneer 92Y22’ (SCN-susceptible soybean sequences) and ‘Pioneer 92Y12’ (SCN-resistant soybean sequences). Plots were managed with conventional tillage as the site was chisel plowed each fall and field cultivated each spring before planting and soil sampling. Weeds and insects were managed with herbicide and insecticide applications as needed. Glyphosate herbicide was applied on 28 June 2013 and 11 June 2014 at 1.14 and 1.42 liters a.i. ha1 respectively. In 2014, lambda-cyhalothrin foliar insecticide (Warrior, Syngenta Crop Protection Inc.) was applied at 0.028 kg a.i. ha1 on August 19 for soybean aphid (Aphis glycines) control. Crops were fertilized such that soil nutrients should not have been a limiting factor for crop yield because the site used in this study was originally established to examine agronomic aspects of corn–soybean rotation when soil nutrients are not limiting. Since site establishment, plots planted to corn received nitrogen application above the recommended rate on a yearly basis while P and K were applied every 3 years or more frequently if needed based on soil testing. In 2013 and 2014, plots planted to corn received nitrogen applications at 224 kg N ha1 in the form of urea with agrotain. This was surface-broadcast without incorporation on 12 June 2013 and 11 June 2014. In 2014, all plots also received P

Within each season (spring, midseason, and fall), data for the 2 years (2013–2014) was combined by treatment. The combined data for each season was analyzed using two-way, split-plot ANOVA. ANOVA models were checked for homogeneity of variance using Levene’s test and for normality of residuals graphically (Cook and Weisburg, 1999; Levene, 1960). When necessary, response variables were transformed to meet these assumptions. For variables with significant crop sequence effects (P  0.05), crop sequence treatment means were separated using Fischer’s protected LSD (P  0.05). All analyses were performed using R version 3.0 (The R Foundation for Statistical Computing, Vienna).

ANOVA (F values) x Crop sequence (C) Year (Y)  C Nematicide (N) NY CN YC  N

ANOVA (F values) Crop sequence (C) Year (Y)  C Nematicide (N) NY CN YC  N

ANOVA (F values) Crop sequence (C) Year (Y)  C Nematicide (N) NY CN YC  N

Bacterivore abundance

Fungivore abundance

Pia z

Pm

Pf

Pi

Pm

Pf

10.04** 1.23 2.02 0.16 0.89 1.04

4.11** 1.25 6.48* 0.10 0.73 1.18

12.05** 1.08 0.01 1.57 0.96 1.55

7.40** 0.93 3.42 0.10 0.97 1.40

5.28** 0.67 31.06** 1.09 1.73 0.63

13.39** 1.27 1.07 0.21 1.01 0.87

Herbivore abundance

Omnivore abundance

Pi

Pm

Pf

Pi

Pm

Pf

13.83** 0.97 52.49** 3.82* 3.90** 1.63

9.21** 1.55 132** 0.48 3.07** 1.00

19.7** 1.17 236** 8.18** 1.32 0.86

1.72 1.83* 3.64 1.75 0.64 1.07

0.54 0.66 1.97 0.09 0.66 0.58

0.84 0.61 14.29** 0.10 1.81 1.10

Predator abundance

Total nematode abundance

Pi

Pm

Pf

Pi

Pm

Pf

0.98 1.02 3.93* 1.04 0.97 1.22

0.42 1.15 3.22 0.09 0.71 1.27

0.63 0.55 0.22 1.64 1.14 1.06

3.39** 1.62 9.60** 0.43 2.33* 0.47

1.44 1.36 67.40** 0.44 2.64** 1.13

2.89** 1.35 70.8** 1.60 3.09** 1.11

*and ** represent significant effects at P  0.05 and P  0.01, respectively. a Pi, Pm, Pf are mean population densities (for nematodes) or values (for indices) prior to planting, at midseason (47–64 days after planting), and at harvest respectively.

3. Results 3.1. Trophic group and total nematode abundances Over 6 seasons in 128 plots, nematodes spanning 64 genera were identified in soil at the site (Supplementary Table 1). Bacterivores were the most abundant feeding type, representing 49.4% of the total nematode population across seasons and plots, followed by herbivores (39.1%), fungivores (10.6%), omnivores (0.71%), and predators (0.09%). Bacterivores with colonizerpersister value 1 (c-p 1) composed 38% of the total nematode population at the site followed by c-p 3 herbivores (21.6%), c-p 2 herbivores (17.2%), c-p 2 bacterivores (11.3%), and c-p 2 fungivores (10.6%). Bacterivores with c-p values of 3 and 4, c-p 4 fungivores, cp 4 and 5 herbivores, c-p 4 and 5 omnivores, and c-p 3 to

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175

Spring

Spring a

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

A

cd cd

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

ab bc a a ab de e e de cd ab de de

A

d d d cd ab ab a ab bc cd a ab

Midseason

abcd bcde abcd ab a abc cdefg

B

Crop Sequence

Crop Sequence

Midseason Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

efg g defg bcde cdef fg efg

bcd

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

B

d d d cd cd bc bc ab a a d abc ab

Fall

Fall bc

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

C ab ab ab a abc

cd e e e e de e e 0

250

500

750

1000 1250 1500 1750

# Bacterivores/100 cm3 soil Fig. 1. Bacterivore populations as influenced by crop sequences in (A) spring, (B) midseason (C) and fall. Values are treatment means – averaged across 4 replicates and 2 nematicide treatments – and standard errors for 2013–2014 combined data. Spring, midseason, and fall are prior to planting, 47–64 days after planting, and at harvest respectively. Mean separation (P  0.05, Fischer’s protected LSD) is within each subfigure and based on transformed values. Cc and Cn are continuous corn with non-Bt and Bt cultivars respectively since 2010; C1 through C5 are 1st- to 5thyear corn after 5 years of soybean; S1 through S5 are 1st- to 5th-year soybean following 5 years of corn; Ss is continuous soybean. Sr is continuous soybean, but with soybean cyst nematode (SCN) resistant cultivar since 2010. All soybeans, except Sr, were susceptible to SCN.

5 predators were also present at the site, but each represented 0.06% or less of the total nematode population. Population dynamics of the economically important plant-parasitic nematodes present at the site – including Heterodera glycines,Pratylenchus, Helicotylenchus, and Xiphinema – are reported in a separate study (Grabau and Chen, 2016a,b unpublished results). Bacterivore population was significantly affected by nematicide at midseason only (Table 2) with density decreased by treatment with nematicide application compared to treatment without nematicide application with densities of 654 and 560 nematodes 100 cm3 soil respectively. Bacterivore populations were significantly affected by crop sequence – combined across nematicide treatments – in all three seasons with populations generally significantly greater in soybean than corn, particularly in fall (Table 2; Fig. 1A–C). Among soybean sequences, there were few significant differences in bactivore population densities (Fig. 1A–C) with some increases in population as time in monoculture

defg efg def

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

C

g fg defg de bc ab a ab cd ab ab 0

100

200

300

400

500

600

# Fungivores/100 cm3 soil Fig. 2. Fungivore populations as influenced by crop sequences in (A) spring, (B) midseason, and (C) fall. Values are treatment means averaged across nematicide treatments and standard errors for 2013–2014 combined data. Refer to Fig. 1 legend for explanation of abbreviations.

increased for sequences in 3 or fewer years of soybean. Among corn sequences, there were also few significant differences including no differences in fall (Fig. 1C), but some decreases in population densities as time in corn monoculture increased in spring and midseason. Fungivore population was significantly affected by nematicide at midseason only (Table 2) with density decreased from 123 to 76 nematodes 100 cm3 soil by nematicide application compared to no nematicide application. Fungivore populations were significantly affected by crop sequence – combined across nematicide treatments – in all three seasons (Table 2). Populations were significantly increased in corn compared to soybean for most sequences although this was more distinct in fall than spring or midseason (Fig. 2A–C). Among corn sequences, fungivore population density was smaller in 1st-year corn than most corn sequences, and greater in middle years of corn than 5th corn year in midseason and fall, but not different among most other sequences. Population densities did not vary significantly across most soybean sequences, although density was significantly greater in 1 year than 4 or more years of SCN-susceptible soybean at midseason. Nematicide applications significantly decreased herbivore populations in all three seasons compared to treatment without

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Z.J. Grabau, S. Chen / Applied Soil Ecology 100 (2016) 172–185

nematicide application (Table 2; Fig. 3A). In both spring and midseason, there were significant crop sequence by nematicide interactions (Table 2). There were significant crop sequence effects for the treatment with nematicide application and the treatment without nematicide application in both spring and midseason (P  0.05, ANOVA), but there were more significant variations across crop sequences without nematicide than with nematicide application (Fig. 3B–E). In both seasons, without nematicide application, population densities were generally greater in or following many years in corn and smaller in or following many years in soybean (Fig. 3B and C). At midseason with nematicide, there were few differences among sequences (Fig. 3E), while before planting, with nematicide, herbivore densities were greater in corn monoculture and smaller in soybean monoculture to some extent (Fig. 3C). In fall, there were significant crop sequence effects (Table 2) and herbivore populations were significantly greater in extended corn monoculture (3 or more years) than all soybean sequences except 1st year soybean (Fig. 3F). Population densities also increased significantly as years in corn increased, from 2 to 4 years of monoculture, but were similar among sequences in 4 or more years of Bt corn. Population densities were similar among sequences in soybean

*

No Nematicide Nematicide

1100 1000

A

Midseason No Nematicide

900

*

800 700

*

600

Crop Sequence

# Herbivores/100 cm3 soil

1200

2–5 years, but smaller in Ss and larger in S1 than most other soybean sequences. Omnivore and predator populations were small at site, averaging 11 and 1 nematode 100 cm3 soil respectively across all plots and seasons (data not shown). In fall, omnivore population was significantly (Table 2) decreased by nematicide application (5 and 12 omnivores 100 cm3 soil, in nematicide and nonnematicide treatments, respectively) in fall. Predator population was significantly (Table 2) decreased by nematicide application compared to without nematicide application (1 and 2 predators 100 cm3 soil) before planting. Neither omnivore nor predator populations were significantly affected by crop sequence (Table 2). Total nematode population densities were significantly decreased by nematicide applications in all three seasons compared to without nematicide application (Table 2; Fig. 4A). In spring, there were no significant crop sequence effects (P > 0.05, ANOVA) for combined nematicide treatments (Table 2) or under individual nematicide treatments (data not shown). There were significant crop sequence by nematicide interactions in midseason and fall (Table 2). In midseason, there were significant (P  0.05, ANOVA) crop sequence effects only without nematicide (data not shown) with population densities significantly greater in Cn and smaller in

500 400 300 200

Spring

Midseason

Fall

B

fg de cde efg def ab abc

D efg cde cde def abcd a abc abc bcde

h ab a

Midseason Nematicide Applied Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

bcd

d bc bc bc a bc ab bc bc bc abc ab 0

cde

E

bc c

200 400 600 800 1000 1200 1400 1600 1800

# Herbivores/100 cm3 soil

g efg abc a

Fall

Spring Nematicide Applied f

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

C

bcde bcde de cde cde ab abcd bcde ef bcde abcd abc a 0

200 400 600 800 1000 1200 1400 1600

# Herbivores/100 cm3 soil

Crop Sequence

Crop Sequence

Spring No Nematicide h

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

fgh gh

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

ef

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

F

f de def de d abc

a ab c d de abc bc 0

200 400 600 800 10001200140016001800

# Herbivores/100 cm3 soil

Fig. 3. Herbivore populations as influenced by nematicide application and crop sequences. Values are treatment means and standard errors for 2013–2014 combined data influenced by (A) nematicide application; (B) crop sequences in spring without nematicide and (C) with nematicide; crop sequences at midseason (D) without nematicide and (E) with nematicide; and (F) crop sequences in fall. Nematicide treatment means are averaged across crop sequences while crop sequence treatment means are averaged across nematicide treatments except where noted. * indicates significant nematicide effect (P  0.05, ANOVA) within the given season. Refer to Fig. 1 legend for explanation of abbreviations.

Z.J. Grabau, S. Chen / Applied Soil Ecology 100 (2016) 172–185

# Nematodes/100 cm3 soil

2400 2200

*

No Nematicide Nematicide

2000

A

1800 1600 1400

*

*

1200

800

Midseason

Fall

Crop Sequence

Fall No Nematicide Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

B

ef f bcdef cdef bcdef bcdef bcd

a ab bcdef def f bcde bc

Fall Nematicide Applied abcd ab ab ab

Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

C

a abc abcd d abcd abcd cd d bcd cd 0

diversity was significantly smaller in Sr than any other sequence while diversity was significantly greater in extended corn monoculture (C4, Cc, and Cn) compared to all sequences following soybean. There were also significant (P  0.05, ANOVA) crop sequence effects in midseason and diversity was significantly greater in corn than soybean for most sequences (data not shown). 3.3. Maturity indices

1000

Spring

177

500 1000 1500 2000 2500 3000 3500 4000

# Nematodes/100 cm3 soil Fig. 4. Total nematode populations as influenced by nematicide application and crop sequences. Values are treatment means and standard errors for 2013– 2014 combined data as influenced by: (A) nematicide application, and crop sequences in fall (B) with nematicide or (C) without nematicide. Nematicide treatment means are averaged across crop sequences while crop sequence treatment means are averaged across nematicide treatments. Refer to Fig. 1 legend for explanation of abbreviations.

C1 and Ss than many other sequences (P  0.05, Fischer’s LSD). In fall, there were significant (P  0.05, ANOVA) crop sequence effects with nematicide and without nematicide application (Fig. 4B and C). With nematicide application, population densities were significantly greater in extended soybean monoculture than many corn sequences (Fig. 4B). In contrast, without nematicide application, population densities were similar among most sequences although significantly smaller in extended soybean monoculture and significantly greater in extended corn monoculture (Fig. 4C). 3.2. Diversity indices The Shannon-–Weaver Diversity Index was significantly affected by nematicide (Table 3) with values decreased by nematicide applications in midseason and fall (1.61 and 1.73 respectively with nematicide) compared to treatment without nematicide application (1.85 in both seasons). There was significant year by crop sequence interaction before planting (Table 3) with significant (P  0.05, ANOVA) crop sequence effects in 2013 but not 2014 (data not shown). Before planting in 2013,

The maturity index was significantly decreased by nematicide application in all three seasons (Tables 3 and 4). The maturity index was also significantly affected by crop sequence in all three seasons and was significantly greater in corn than soybean for most sequences (Tables 3 and 4). In each season, values were greater in 1st-year soybean than other soybean sequences, but there were few other differences among soybean sequences. Among corn sequences, values were smaller in initial than later years in corn monoculture although the significance of specific contrasts varied by season. MI25, maturity index with enrichment opportunists excluded, was significantly decreased (P  0.05, ANOVA) by nematicide application in fall (values of 2.04 and 2.08 with and without nematicide respectively) and spring 2014 (values of 2.10 and 2.16 with and without nematicide respectively). There were significant (P  0.05) crop sequence effects only in midseason and only for the treatment without nematicide application (data not shown). In midseason, without nematicide, MI25 was similar among most sequences, but significantly greater in C1 and Ss than most corn sequences (data not shown). There were significant nematicide by year interactions for the plant-parasite index (PPI) in midseason and fall (Table 3). In most seasons, PPI was decreased by nematicide compared to no nematicide treatment, but values were increased by nematicide application in midseason 2014 (Fig. 5A). There was significant nematicide by crop sequence interaction in midseason (Table 3) with significant (P  0.05, ANOVA) crop sequence effects with nematicide or without nematicide application (Fig. 5B and C). With nematicide application, PPI was significantly greater in extended SCN-susceptible soybean monoculture (4 or more years) than some corn sequences, but without nematicide application there were few differences. PPI was significantly smaller with SCN-resistant soybean than most SCN-susceptible soybean sequences under either nematicide treatment. With combined nematicide treatments in spring and fall, there were significant crop sequence effects (Table 3) and PPI values were also significantly smaller under SCN-resistant soybean monoculture than most other crop sequences (Fig. 5D and E). Values also tended to be greater in or following extended soybean monoculture and smaller in or following extended corn monoculture. 3.4. Food web indices The enrichment index was significantly increased by nematicide applications compared to without nematicide application and affected by crop sequence – averaged across nematicide treatments – in all three seasons (Tables 3 and 4). In each season, the enrichment index was significantly greater under soybean than corn for most sequences (Tables 3 and 4). Values were similar across most soybean sequences, but smaller in or entering 1st-year soybean than other soybean sequences. Among corn sequences, values generally decreased as time in corn increased within the first 3 years in or, before planting, following corn, but did not decrease with further increases in time in corn. In both spring and midseason, the basal index was significantly decreased by nematicide applications (Table 3; Fig. 6A). In midseason, there was significant crop sequence by nematicide

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Table 3 Effects of crop sequence and nematicide on nematode populations and nematode community indices for 2013–2014 combined data. Shannon–Weaver diversity Index a

ANOVA (F values) x Crop Sequence (C) Year (Y) x C Nematicide (N) NxY CxN YxCxN

Maturity index

Vi z

Vm

Vf

Vi

Vm

Vf

Vi

Vm

Vf

1.93* 2.19* 1.27 1.42 0.54 1.39

3.47** 0.62 36.04** 0.28 1.64 1.28

1.22 0.76 7.24** 6.13* 0.62 1.12

9.19** 1.72 9.44** 0.00 0.91 0.71

3.64** 0.53 25.7** 1.82 1.09 0.97

40.0** 1.36 8.45** 0.73 0.91 1.04

1.06 1.40 3.58 4.45* 1.13 1.29

2.05* 1.39 2.65 0.65 2.01* 2.09*

1.04 0.74 7.00** 0.42 0.87 1.14

Plant-parasite index

ANOVA (F values) Crop sequence (C) Year (Y) x C Nematicide (N) NxY CxN YxCxN

Enrichment index

Basal index

Vi

Vm

Vf

Vi

Vm

Vf

Vi

Vm

Vf

7.00** 0.85 0.12 0.31 1.69 1.11

5.74** 1.82 0.66 27.0** 2.69** 0.84

7.96** 1.52 36.5** 10.1** 1.39 0.42

7.13** 1.04 4.85* 0.86 0.92 0.48

4.61** 0.22 14.0** 1.15 1.61 0.61

37.8** 1.43 3.83* 0.57 0.88 1.16

6.84** 1.03 4.01** 1.50 0.93 0.53

5.19** 0.42 14.2** 0.04 1.96** 1.22

35.6** 1.22 2.89 0.63 0.93 1.04

Structure index

ANOVA (F values) Crop sequence (C) Year (Y) x C Nematicide (N) NxY CxN YxCxN

MI25

Channel index

FBPP

Vi

Vm

Vf

Vi

Vm

Vf

Vi

Vm

Vf

2.43** 1.85* 6.65* 2.79 0.87 0.88

1.65 0.70 3.05 0.03 1.33 1.07

0.32 0.49 8.71** 0.82 1.53 1.22

18.37** 1.08 5.59* 0.04 0.38 0.82

9.89** 0.36 23.64** 2.45 1.84* 0.43

52.90** 1.21 0.56 0.43 1.63 0.66

19.40** 0.95 40.91** 1.32 1.62 1.48

15.16** 0.66 74.68** 0.18 1.86* 0.41

34.23** 1.68 211.4** 9.55** 1.14 1.08

* and ** represent significant effects at P0.05 and P0.01 respectively. a Vi, Vm, Vf are values prior to planting, at midseason (47-64 days after planting), and at harvest respectively.

interaction (Table 3) with significant crop sequence effects both with nematicide and without nematicide application (P  0.05, ANOVA). Values were significantly greater in many corn than many Table 4 Effects of crop sequence and nematicide on maturity index and enrichment index for 2013 and 2014 combined data. Maturity index Via,b

Enrichment index

Vm

Vf

Via

Vm

Vf

Crop sequencec Cn 1.49ab Cc 1.47ab C1 1.27de C2 1.36cd C3 1.41bc C4 1.55a C5 1.47ab S1 1.49ab S2 1.29de S3 1.26e S4 1.26de S5 1.29de Ss 1.32cde Sr 1.26de

1.49a 1.46ab 1.29cde 1.42abc 1.49a 1.43abc 1.33bcde 1.40abcd 1.26de 1.26de 1.25e 1.26e 1.26de 1.20e

1.72a 1.76a 1.53c 1.64b 1.74a 1.77a 1.71ab 1.55c 1.35de 1.32e 1.33de 1.41d 1.34de 1.38de

85de 84e 92ab 90abc 88bcd 83e 87cde 84e 92a 93a 93a 92ab 91ab 92ab

86d 86d 94ab 88cd 84d 88cd 93abc 89bcd 94a 94a 95a 94ab 95a 95a

69fg 68fg 80c 75de 69fg 67g 72ef 79cd 88ab 91a 90a 86b 89ab 89ab

Nematicide Not applied Applied

1.39A 1.30B

1.57A 1.51B

88B 90A

90B 92A

78B 80A

1.40A 1.34B

a Vi, Vm, and Vf are mean values prior to planting, at midseason (64 and 45 days after planting in 2013 and 2014), and at harvest respectively. b Different letters in the same column indicate significant difference (Fischer’s LSD, P  0.05) between transformed mean values of the same factor (nematicide or rotation). c Cc and Cn are continuous corn with non-Bt and Bt cultivars since 2010 respectively; C1 through C5 are 1st- to 5th-year corn after 5 years of soybean; S1 through S5 are 1st- to 5th-year soybean following 5 years of corn; Ss and Sr are continuous soybean with SCN-susceptible and resistant cultivars respectively since 2010. All soybeans, except Sr, were susceptible to SCN.

soybean sequences although more contrasts were significant for treatment with nematicide than treatment without nematicide (Fig. 6B and C). In spring and fall there were significant crop sequence effects – averaged across nematicide treatments – on the basal index (Table 3) with values larger in corn than soybean for most sequences, particularly in fall (Fig. 6D and E). Values were greater in 1st-year soybean than any other soybean sequence, but there were no differences among other soybean sequences. Values were greater for sequences later than sequences earlier in corn monoculture. The structure index was low with average values across treatments of 17, 27, and 9 in spring, midseason, and fall respectively. The structure index was significantly (Table 3) decreased by nematicide application compared to no nematicide in spring (values of 18.8 and 14.7 respectively) and fall (values of 11.5 and 6.8 respectively). There was significant crop sequence by year interaction before planting (Table 3). There were only significant crop sequence effects in 2013 (P  0.05, ANOVA) with the structure index significantly (P  0.05, Fischer’s LSD) greater in sequences following 5 or more years of corn monoculture and smaller in Sr than most sequences. The channel index was significantly decreased by nematicide application in spring and midseason (Table 3; Fig. 7A). There was significant crop sequence by nematicide interaction in midseason (Table 3) with significant crop sequence effects both with nematicide and without nematicide application (Fig. 7B and C). At midseason, the channel index was significantly greater in most corn than most soybean sequences, although more contrasts were significant with nematicide than without nematicide application (Fig. 7B and C). In spring and fall, with nematicide treatments combined, there were significant crop sequence effects (Table 3) with values significantly greater in corn than soybean sequences, excluding 1st-year sequences (Fig. 7C and D). Values were

Z.J. Grabau, S. Chen / Applied Soil Ecology 100 (2016) 172–185

Spring

A

2.7

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g

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def

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Plant Parasite Index

Sr

ef

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abcde

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abc ab cde cde abcd cde bcde bcde bcde ef a de abcde

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a

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abc

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def

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cde

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ef

C2

def a

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def

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3.2

Plant Parasite Index

Crop Sequence

Crop Sequence

Midseason No Nematicide Sr Ss S5 S4 S3 S2 S1 C5 C4 C3 C2 C1 Cc Cn

C1

0.0

0.4

Plant Parasite Index

0.8

1.2

1.6

2.0

2.4

2.8

3.2

Plant Parasite Index

Fig. 5. Plant-parasite index as influenced by nematicide application and crop sequences. Subfigure (A) is treatment means and standard errors in spring, midseason, and fall in 2013 and 2014 as influenced by nematicide application (averaged across crop sequences). In remaining subfigures, values are treatment means and standard errors for 2013– 2014 combined data as influenced by: crop sequences at midseason (B) without nematicide and (C) with nematicide; and crop sequences in (D) spring, and (E) fall. Crop sequence treatment means are averaged across nematicide treatments. Refer to Fig. 1 legend for explanation of abbreviations.

significantly greater in 1st-year soybean than other soybean sequences, but similar among sequences in 2 or more years of soybean. Channel index values increased significantly as years in corn increased for sequences in or entering 1–3 years in corn, and values were smaller in 5th-year corn than peak values. FBPP, fungivore and bacterivore divided by herbivore population densities, was significantly increased by nematicide application in all three seasons (Table 3; Fig. 8A). There was significant crop sequence by nematicide interactions in midseason (Table 3) with significant crop sequence effects with nematicide or without nematicide application (Fig. 8B and C). At midseason, FBPP was generally greater in soybean than corn with more differences for treatment without than treatment with nematicide. Values were significantly greater in SCN-resistant soybean sequence than any other sequence with nematicide, but without nematicide only greater than most other sequences. Before planting and in fall, with nematicide treatments combined, there were significant crop sequence effects (Table 3; Fig. 8D and E). In fall, values were greater in 2 or more years of soybean than in corn or 1st-year soybean (Fig. 8E). Before planting, values were greater in 2 or more years of soybean than 4 or more years of corn for most contrasts, and greater in SCN-resistant soybean than any other sequence (Fig. 8D).

Before planting and in fall, values were also smaller in 4 or more than 3 or fewer years of corn monoculture. 4. Discussion Nematicide was consistently effective against its target, herbivores, and decreased nematode populations for nearly a year following nematicide application. This long-lasting effect may have been enhanced because application was repeated yearly. However, nematicide also reduced populations of non-target, beneficial nematodes including bacterivores, fungivores, omnivores and predators in various seasons. The loss of the ecological services provided by these nematodes and ecologically similar organisms contributes to the cost of nematicide application, although this loss is not easily quantified (Anderson et al., 1983; Bongers, 1990; Chen and Ferris, 1999; Ferris et al., 1998, 2001). Prior information on the impact of aldicarb nematicide application on the nematode community is limited, but in another study aldicarb application reduced populations of free-living nematodes in most fields it was applied (Smolik, 1983). In other studies, application of other granular nematicides also reduced populations of free-living nematodes, particularly bacterivores and

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No Nematicide Nematicide

20

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de

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de

18

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Crop Sequence

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A

22

14 12

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Basal Index

de cde de bcde

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de abcd ab a de

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a abcd bcdef f cdef ef cdef

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abc

Crop Sequence

Crop Sequence

Midseason No Nematicide

20

22

Basal Index

0

3

6

9

12 15 18 21 24 27 30 33 36

Basal Index

Fig. 6. Basal index as influenced by nematicide application and crop sequences. Values are treatment means and standard errors for 2013–2014 combined data as influenced by (A) nematicide application; crop sequences at midseason (B) without nematicide and (C) with nematicide; (D) crop sequences in spring (E); and crop sequences in fall. Nematicide treatment means are averaged across crop sequences while crop sequence treatment means are averaged across nematicide treatments. Refer to Fig. 1 legend for explanation of abbreviations.

fungivores (Chelinho et al., 2011; Pen-Mouratov and Steinberger, 2005; Wada et al., 2011). Fumigant nematicides, which are applied across the entire soil and target a broad spectrum of organisms, affected most trophic groups (Sanchez-Moreno et al., 2010; Timper et al., 2012; Wang et al., 2006). Bacterivore and fungivore populations were impacted for shorter duration during the present study than herbivores, a trend observed with application of some pesticides (Timper et al., 2012; Wang et al., 2006). This suggests bacterivores and fungivores were more resilient than herbivores perhaps partially due to the former group’s r reproductive strategy. The release of organic matter from the death of other organisms due to nematicide application may have stimulated microbial growth increasing food resources for bacterivores and fungivores and helped counteract the initial decrease in fungivore and bacterivore populations from nematicide application. Diversity was decreased by nematicide application through one growing season and is similar to other results with granular nematicide (Pen-Mouratov and Steinberger, 2005) and fumigants (Ettema and Bongers, 1993; Sanchez-Moreno et al., 2010; Wang et al., 2006). Decreased maturity and structure indices with nematicide application suggest nematicide application disturbed

the soil food web a full year after application. Similarly, in other studies, both granular nematicides (Pen-Mouratov and Steinberger, 2005) and fumigants (Sanchez-Moreno et al., 2010; Timper et al., 2012; Wang et al., 2006) reduced soil community maturity. Based on increased enrichment index values and decreased basal index values, nematicide application enriched the soil ecosystem – which is consistent with an influx of resources from decaying organisms – and shifted the ecosystem away from a basal trajectory. In other studies, carbofuran granular nematicide (Chelinho et al., 2011) and the fumigant 1,3-dichloropropene (1,3-D) enriched the nematode community (Sanchez-Moreno et al., 2010) while 1,3-D in combination with aldicarb granular nematicide did not affect soil food web enrichment (Timper et al., 2012). Based on the faunal profile (Ferris et al., 2001), in the present study, the agrosystem was classified as disturbed (enrichment index greater than 50, structure index less than 50) with or without nematicide application. Based on decreases in the channel index with nematicide application, nematicide application shifted decomposition pathways toward bacterial rather than fungal channels. This is consistent with enrichment of the ecosystem under nematicide application and with the results of other studies (Timper et al.,

Z.J. Grabau, S. Chen / Applied Soil Ecology 100 (2016) 172–185

Spring

A

18

No Nematicide Nematicide

16

d

Sr

d

S5

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S4 12 10 8

*

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Channel Index

181

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6

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d d cd bcd

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bcd a bcd a

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d d d d abc c ab

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ab

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Crop Sequence

Midseason No Nematicide

18

Channel Index

0

4

8

12

16

20

24

28

32

36

40

Channel Index

Fig. 7. Channel index as influenced by nematicide application and crop sequences. Values are treatment means and standard errors for 2013–2014 combined data as influenced by (A) nematicide application; crop sequences at midseason (B) without nematicide and (C) with nematicide; (D) crop sequences in spring; and (E) crop sequences in fall. Nematicide treatment means are averaged across crop sequences while crop sequence treatment means are averaged across nematicide treatments except where noted. Refer to Fig. 1 legend for explanation of abbreviations.

2012; Wang et al., 2006). Similarly, nematicide application shifted FBPP ratio toward free-living nematodes again showing nematicide application was more effective against herbivores than freeliving nematodes. In the initial years in a particular crop – the number of years varied by population or indicator – most nematode populations and indices shifted. Increasing values or densities in one crop generally corresponded with decreasing values or densities in the other crop. This lead to different nematode community characteristics between the two crops, particularly in monoculture. For most nematode indicators, and thus aspects of soil ecology, after a certain number of years in a particular crop, which varied by indicator, values stopped changing with increasing years in monoculture or the rate of change was dramatically reduced. This suggests the corresponding aspect of the soil community had reached an equilibrium status for that particular crop within the given agricultural environment and further increases in monoculture would not substantially shift that aspect of the soil community. Within this context, temporal variations in nematode populations and soil food web status still occurred. These states of equilibrium did not equate to an advanced stage of ecological succession as the site was intensively managed and every cropping

sequence was classified as a disturbed system (enrichment index greater than 50, structure index less than 50) according to the faunal profile (Ferris et al., 2001). Other long-term crop sequence experiments that examined the nematode community did not have comparable designs to this study, so they cannot be used to confirm these trends (Djigal et al., 2012; Rahman et al., 2007). The extent of differences between cropping systems varied for different nematode indicators of soil ecology and fluctuated by season, but corn and soybean cropping systems had distinct characteristics. In particular, herbivore populations were greater in corn compared to soybean monoculture. Population trends of potentially economically important plant-parasitic nematodes at this site are discussed in a separate study (Grabau and Chen, 2016a,b unpublished results). While corn was better than soybean for fungivore development, soybean was better than corn for bacterivore development based on populations of these nematodes. As expected given these trends in bacterivore and fungivore populations, corn monoculture promoted fungal rather than bacterialbased decomposition pathways compared to soybean monoculture based on greater channel index values. These trends also suggest corn systems were more conducive to fungal growth which is associated with a more stable or structured ecosystem (Ferris et al.,

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Z.J. Grabau, S. Chen / Applied Soil Ecology 100 (2016) 172–185

Spring

5.2

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5

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FBPP

Fig. 8. FBPP as influenced by nematicide application and crop sequences. Values are treatment means and standard errors for 2013–2014 combined data as influenced by (A) nematicide application; (B) crop sequences in midseason without nematicide and (C) with nematicide; (D) crop sequences in spring; and (E) crop sequences in fall. Nematicide treatment means are averaged across crop sequences while crop sequence treatment means are averaged across nematicide treatments except where noted. Refer to Fig. 1 legend for explanation of abbreviations.

2001). In contrast, soybean systems were more conducive to bacterial growth, based on greater bacterivore populations, and had a more highly enriched food web. Studies investigating crop residue decomposition in soil reinforce this conclusion as they have demonstrated that bacteria population densities are greater in decomposing soybean than corn shoot residue, fungal population densities are greater in decomposing corn than soybean residue (Broder and Wagner, 1988), and fungi are the predominant group involved in corn stalk residue decomposition (Wagner and Broder, 1993). Since bacteria have a lower C:N ratio than fungi (Anderson et al., 1983; Chen and Ferris, 1999; Woods et al., 1982), these trends in microbivore populations also suggest system inputs had a smaller C:N ratio under soybean than corn as demonstrated elsewhere (Halvorson and Schlegel, 2012; Salvator and Sabbe, 1995). Corn residue C:N ratio is also greater following corn than soybean (Gentry et al., 2001) which may have accelerated the shift toward fungivores under corn monoculture. Additionally, corn takes up more nitrogen from the soil than soybean – which is a nitrogenfixing legume – on a per hectare basis (Halvorson and Schlegel, 2012) resulting in nitrogen immobilization (Salvator and Sabbe, 1995), which may also contribute to greater C:N ratio in the soil under corn than soybean. In another study, adding inputs with

greater C:N ratio also increased fungivore and decreased bacterivore population compared to adding inputs with smaller C:N ratio (Ferris et al., 1996). The greater proportion of recalcitrant components – such as hemicellulose, cellulose, and lignin – compared to soluble components in corn compared to soybean shoot residue (Broder and Wagner, 1988) may also contribute to greater fungivore populations in corn since fungi are more predominant in decomposition stages involving these components (Broder and Wagner, 1988; Wagner and Broder, 1993). In addition to differences in composition, aboveground residue volume is also much greater in corn than soybean systems (Halvorson and Schlegel, 2012) and this greater volume of substrate for microbial growth may have contributed to nematode population differences. Differences in the basal and enrichment indices between cropping systems were related to greater population of enrichment-opportunist bacterivores under soybean than corn systems and suggested soybean monoculture created more enriched conditions than corn monoculture (Ferris et al., 2001). These trends in enrichment, fungivore populations, and bacterivore populations suggest nutrient mineralization by microorganisms may play a role in plant growth benefits from crop rotation, particularly for corn. Bacterivores, fungivores, and the microbial

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food sources these nematodes are indicators of are known to mineralize nutrients with bacteria and bacterivores having a larger contribution (Chen and Ferris, 1999; Ferris et al., 1998; Holtkamp et al., 2011; Rosswall and Paustian, 1984; Woods et al., 1982). This suggests nutrient mineralization may have been increased in soybean compared to corn production since bacterivore populations were also increased in soybean. In turn, increased nutrient mineralization following soybean could play a role in increasing corn yield in corn–soybean crop rotation. Other studies have suggested increased nutrient mineralization following soybean compared with following corn contributes to benefits of corn– soybean rotation for corn (Gentry et al., 2001; Green and Blackmer, 1995). SCN-susceptible soybean monoculture shifted balance toward free-living nematodes versus herbivores compared to corn monoculture based on FBPP (Wasilewska,1989). In particular, this shift was a result of increased herbivore populations and decreased bacterivore populations, which were larger than fungivore populations, in corn compared to soybean systems. Ecosystems skewed toward fungivores and bacterivores may be more healthy as the former provide beneficial services to the ecosystem while herbivores are generally detrimental (Barker and Olthof, 1976; Chen and Ferris, 1999; Ferris et al., 1998; Wasilewska, 1989). Additionally, SCNresistant soybean strongly shifted balance toward free-living nematodes compared with other systems due to control of the major plant-parasitic nematode in soybean, SCN. Corn monoculture created a more diverse system than soybean monoculture, based on the Shannon-Weaver diversity index (Neher and Darby, 2009), although this was not consistent across seasons. Maturity index values were generally smaller in soybean than corn monoculture which reflects the relative increases in bacterivore populations, which were primarily colonizer-persister value 1 (c-p 1) at this site (Bongers, 1990), and decreases in fungivores population, primarily c-p 2 in soybean monoculture. Possible explanations for these population trends were discussed above and these differences in the maturity index suggest corn systems were more mature and less disturbed than soybean systems (Bongers, 1990). In contrast, based on the structure index, structure at the top of the soil food web was generally unaffected by crop sequence. Overall, based on small structure index values and small omnivore-predator populations, the soil food web lacked structure in these cropping systems which is similar to other agricultural systems (Sanchez-Moreno et al., 2010). Impacts of cropping system on MI25 were minimal suggesting differences in maturity were driven by disturbance through enrichment since enrichment opportunists are excluded from MI25. Since enrichment opportunists constituted a large portion of bacterivores at the site, this suggests increase of bacterivores in soybean compared to corn drove these differences in soil ecosystem maturity. Based on increased plant-parasite index values, SCN-susceptible soybean monoculture favored development of c-p 3 – primarily yield-damaging herbivores – over c-p 2 herbivores—primarily herbivores with minimal known impact on yield herbivores (Bongers, 1990). In contrast, corn monoculture shifted ratio toward non-damaging c-p 2 herbivores, particularly in fall. Additionally, much smaller plant-parasite index values in SCN-resistant soybean than other cropping systems reflects control of the major plantparasitic nematode in soybean at the site, SCN, which shifted balance strongly toward c-p 2 herbivores. Other studies have suggested the plant-parasite index may be used as an indicator of disturbance (Bongers et al., 1997; Bongers and Ferris, 1999; Neher and Campbell, 1996), but results of this study suggest that in agricultural systems it may be more appropriate to use this index as an indicator of the dynamics between host-adapted and generalist herbivores.

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There are few published studies on the influence of crop rotation on the nematode community, and therefore few reference points for this study. A relatively similar study is a long-term experiment in Madagascar that included a comparison of corn monoculture with annual rotations involving corn, soybean, and common bean with different conservation practices in each rotation (Djigal et al., 2012). In the long-term study in Madagascar, the soil ecosystem was much different in corn monoculture than annual rotations involving corn, soybean, and common bean. In that study, corn monoculture was more enriched, disturbed and unstructured than other systems, but different conservation practices – including cover crops – were used in each rotation, so ecosystem differences could not be attributed exclusively to rotation type (Djigal et al., 2012). Studies involving other crop rotations also suggested that different crops drive nematode populations, particularly fungivores and bacterivores, and affect the soil ecosystem in different ways (Briar et al., 2012; Carter et al., 2009; Osler et al., 2000; Rahman et al., 2007). In the present study, nematicide application reduced differences among crop sequences for some nematode indicators including herbivore population, total nematode population, MI25, and FBPP, particularly at midseason when nematicide was more effective. In this case, increased nematode populations without nematicide application made differences across crop sequences more distinct. In contrast, there were more differences among crop sequences with nematicide application than without nematicide application for the basal index, channel index, and the plant-parasite index again generally at midseason. In this case, decrease in basal index and channel index and increase in the plant-parasite index under soybean was intensified by nematicide application. 5. Conclusions In summary, nematicide application negatively impacted soil ecology, as indicated by the nematode community, by disturbing the soil ecosystem and reducing populations of beneficial organisms, as indicated by reductions in bacterivore and fungivores populations. Crop sequences influenced the nematode community and soil ecology with corn systems distinct from soybean systems particularly after initial years in crop monoculture. Soybean promoted a more enriched, disturbed ecosystem shifted toward bacterial decomposition pathways compared to corn. Corn promoted a more diverse, mature ecosystem shifted toward fungal decomposition pathways compared to soybean. These differences between corn and soybean systems demonstrate that each crop has a distinct impact on the soil ecosystem. Acknowledgements This research was partially supported by Minnesota Soybean Producers Check-off Funding through the Minnesota Soybean Research and Promotion Council and Minnesota Agricultural Experiment Station. Thanks to Cathy Johnson, Wayne Gottschalk, and Jeff Ballman for technical assistance. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j. apsoil.2015.12.016. References Anderson, R., Gould, W., Woods, L., Cambardella, C., Ingham, R., Coleman, D., 1983. Organic and inorganic nitrogenous losses by microbivorous nematodes in soil. Oikos 40, 75–80.

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