Relationship between soil alteration index three (AI3), soil organic matter and tree performance in a ‘Cripps Pink’/M7 apple orchard

Abstract

Alteration index three (AI3), which calculates the balances between three microbially-secreted enzymes, potentially enables differences between soils due to contrasting management practices to be quantified in relative terms. The ability of AI3 to distinguish between apple orchard soils under conventional and organic production protocols, and to reflect tree performance, were tested in a maturing ‘Cripps Pink’/M7 apple orchard. Activities of β-glucosidase, phosphatase and urease were determined colourimetrically in extracts of tree-row top-soils (0–15 cm) taken during September and January over five consecutive seasons. Soil organic matter content was determined by dichromate oxidation. Stem circumference and yield were measured manually. AI3 correlated significantly (p = 0.05) with soil organic matter, yield and yield efficiency. AI3 may thus be a useful indicator of relative apple tree performance under organic and conventional soil surface management practices.

Keywords:

• AI3
• compost
• enzyme activity
• organic
• straw mulch

Organic (ORG) orchard floor management protocols, using compost as a nutrient source and mulches to control weeds, differ in their effects on soil parameters and apple tree performance from conventional (CON) practices using synthetic fertilisers and herbicides (Wooldridge et al. 2013a, 2013b). Whether ORG and CON practices differ in their effects on the activities of soil enzymes has not been tested in Western Cape apple orchards. Neither has it been established whether enzyme activity ratios bear any relationship to tree performance.

Alteration index three (AI3) quantifies the balance between three microbially-secreted soil enzymes and is sensitive to alterations in soil characteristics caused by management practices (Puglisi et al. 2006). Alteration of the soil, whether by over-utilisation or other detrimental practices, results in AI3 values that are higher than those of control soils (Puglisi et al. 2006 and references therein). AI3 is potentially useful for determining soil quality in temperate grasslands (Paz-Ferreiro et al. 2009), and as an index of soil degradation due to agricultural practices (Bastida et al. 2008).

To clarify the effects of ORG and CON treatments on soil enzyme activity ratios in orchard soils, AI3 was determined in extracts obtained from two CON and three ORG treatments (Table 1) in the latter stages of an ongoing field trial in a ‘Cripps Pink’/M7 apple orchard on a sandy loam soil in the Elgin area (Wooldridge et al. 2013a). Treatments were applied to nine-tree plots replicated in four randomised blocks from September 2003 (beginning of the third growth season) to January 2011.

Table 1: Conventional (CON) and organic (ORG) soil surface management treatments applied in a ‘Cripps pink’/M7 trial orchard

Treatment	Work row		Tree row (drip line to drip line)
	Tree dormant	Tree active
T1 (CON)	Weed cover	Cover mown, natural die-back in summer	Chemical weed control^Table Footnotea + inorganic N^Table Footnoteb
T2 (CON)	Cover crop^Table Footnotec	Cover crop^Table Footnotec, chemical weed control in spring^Table Footnotea	Chemical weed control^Table Footnotea + inorganic N^Table Footnoteb
T3 (ORG)		Permanent straw mulch^Table Footnoted, hand weeded	(Compost + mulch)^Table Footnotee; hand weeded
T4 (ORG)	Cover crop^Table Footnotec	Cover crop^Table Footnotec, mown, natural die-back in summer	(Compost + mulch)^Table Footnotee + compost tea^Table Footnotef; hand weeded
T5 (ORG)	Weed cover	Cover mown, natural die-back in summer	(Compost + mulch)^Table Footnotee; hand weeded

a Roundup 360SC^® applied at 5 litres ha^-1 (3%) in September and January

b Nitrogen as limestone ammonium nitrate (LAN) applied at full bloom, six weeks after full bloom and after harvest at rates of 9.2 g N, 5.2 g N and 18.0 g N tree^-1, respectively, in seasons 2003/04 to 2007/08. Applied after harvest only, at 18.0 g N tree^-1, in 2008/09 and 2009/10. Applied to the tree row in a continuous strip 100 cm wide

c Rye (Secale cereale L. cv. Henog) and oats (Avena sativa L. cv. Pallinup) planted in tilled soil (0-10 cm) after leaf senescence in alternate seasons. Nitrogen (as LAN) applied to the cover crops and weed covers at 14 kg N ha^-1 in June

d Wheat or oat straw (mulch) applied as a 5 cm layer in a continuous strip, c. 100 cm wide, along the tree row. Mulch applied at 3.2 kg straw tree^-1. Work rows covered with a 5 cm layer of straw mulch. Mulches replenished annually in October

e Compost applied at 20 kg per tree^-1 (128 g N, 3.7 g P, 6.3 g K tree^-1) in spring and autumn 2003/04, 10 kg tree^-1 in spring and autumn 2004/05 to 2006/07, and 10 kg tree^-1 in spring 2008 and 2009. Spread in a continuous strip, c. 1 m wide, along the tree row. Covered with a 5 cm layer (c. 3.2 kg per tree^-1) of wheat or oat straw (mulch). Mulch reapplied annually, in October

f Compost tea (aerated, aqueous, microbial digest) surface-applied around each tree at 1 litre tree^-1 per four-week interval from full bloom to leaf senescence. Principal components (mg ml^-1): P, 7.5; K, 190; Ca, 51; Mg, 34.6; Na, 163. Nitrogen: 0.06%

Composite soil samples were taken from the zone of highest white feeding root concentration (c. 0–15 cm depth) on both sides of the tree row, beneath the canopy drip zone, from September 2006, when the trees were approaching maturity to January 2011. The soil organic matter (SOM) contents and chemical characteristics of these samples were determined by Wooldridge et al. (2013a). Activities of β-glucosidase, urease and phosphatase were assessed colourimetrically in extracts from the 0–15 cm drip-zone soil samples (Tabatabai and Bremner 1969; Eivazi and Tabatabai 1988; Kandeler and Gerber 1988). AI3 values were calculated with the equation:

AI3 = (7.87 × β-glucosidase) − (8.22 × phosphatase) – (0.49 × urease)

where enzyme activities were expressed in micromoles of, respectively, p-nitrophenyl-β-D-glucoside and p-nitrophenylphosphate per gram of soil per hour, and micrograms of urea per gram of soil per hour.

Stem circumferences were measured 40 cm above ground level using a flexible tape at the end of seasons 2003/04 to 2009/10 (Wooldridge et al. 2013b). Yields were determined annually, and yield efficiencies (yield in kg cm^-2 stem cross-sectional area) were calculated for seasons 2006/07 to 2010/11 (Wooldridge et al. 2013b). Data were subjected to analysis of variance using SAS 9.1.3 (SAS Institute 2008). Least significant difference values were calculated at p = 0.05 (Student's t-test) to facilitate comparison between treatment means. Pearson correlation coefficients (r), correlating AI3 with the SOM, stem circumference, yield and yield efficiency data of Wooldridge et al. (2013a, 2013b) were derived using the CORR procedure of SAS 9.1.3.

Over all of the seasons in which treatments were applied (2003–2010), SOM contents in ORG treatments T3, T4 and T5 were greater than in CON treatments T1 and T2 (Table 2). Average increases in stem circumference over this period were also greater in T3, T4 and T5 than in T1 and T2 due to higher levels of phosphorus and potassium, and lower acidity, in the ORG than the CON soils (Wooldridge et al. 2013b). Over the period 2006 to 2010, however, when the trees were mature, increases in stem circumferences did not differ significantly between treatments. Percentage increase in stem circumference in T3 and T4 were nevertheless greater than in T1. Average yields between 2006 and 2010 were greater in T1 than in T3, T4 and T5. Over the same period, average yield efficiencies in T1 and T2 exceeded those in T3, T4 and T5. September AI3 values in T4 and T5 were lower than in T1 and T2. January AI3 values were lower in T3, T4 and T5 than in T2. That the AI3 value in T2 tended to be lower than in T1 may have been due to carry-over of the effects of work-row tillage in T2 into the drip zone from which the samples were obtained. According to Bergstrom et al. (1998), tillage may affect enzyme activities. Similarly, Puglisi et al. (2006) found that control soils were characterised by lower AI3 values than soils that had been altered by detrimental management practices. The altered soils were inferred to be less able to support plant growth than the control soils.

Table 2: Effect of conventional (CON) and organic (ORG) management on soil alteration index three (AI3), and on soil organic matter (SOM) content, stem circumference, yield, yield efficiency (kg cm^-2 stem cross-sectional area) in a ‘Cripps pink’/M7 apple orchard. Values in the same column followed by the same letter do not differ significantly at p = 0.05

Treatment	SOM 2003-2010 (%)^Table Footnote1	Increase in stem circumference 2006-2010 (cm)^Table Footnote2	Increase in stem circumference 2003-2010 (cm)^Table Footnote2	Increase in stem circumference 2006-2010 (%)^Table Footnote2	Average yield 2006-2010 (t ha^-1)^Table Footnote2	Yield efficiency 2006-2010 (kg cm^-2)^Table Footnote2	AI3 September 2007-2010	AI3 January2008-2011
T1 (CON)	1.06^d	12.60^b	6.3^a	11.5^b	38.71^a	0.386^a	-23^ab	-47^ab
T2 (CON)	2.03^cd	12.58^b	6.5^a	12.6^ab	32.59^ab	0.363^a	-20^a	-43^a
T3 (ORG)	3.22^ab	17.15^a	8.0^a	15.0^a	29.30^bc	0.218^b	-33^bc	-51^b
T4 (ORG)	3.54^a	16.05^a	7.6^a	14.8^a	22.51^c	0.186^b	-36^c	-51^b
T5 (ORG)	3.45^a	15.90^a	6.8^a	13.9^ab	21.74^c	0.168^b	-36^c	-51^b

1 Tree row including drip zone. Data of Wooldridge et al. (2013a)

2 Data of Wooldridge et al. (2013b)

Yields and yield efficiencies were higher in the treatments in which stem circumferences over the whole period of treatment applications (2003–2010) were lowest, which support the principle that vegetative growth in deciduous fruit trees is inversely related to fruitfulness (Jerie et al. 1989). Low yields and yield efficiencies in the present trial were attributed to high mineral nutrient levels, notably in the ORG treatments, which led to excessive vegetative growth in the young trees, necessitating girdling (Wooldridge et al. 2013b).

The observation that the September and January AI3 values were less well correlated than the September and January SOM contents (Table 3) implies that AI3, which reflects microbiological activity, is more variable within seasons than SOM, probably because SOM indicates the abundance of carbon in living organisms and in their no-longer living residues. AI3 therefore appears to be more sensitive to environmental conditions than SOM. Likewise, Jin et al. (2009) found that enzyme activities in soils vary within seasons. AI3 was more highly correlated with SOM in September than in January. AI3 correlated significantly with neither stem circumference nor percentage increase in stem circumference over the period 2006–2010, either in September or January. Yield and yield efficiency correlated significantly with AI3 in September, but not January, indicating that sampling for enzyme assays in apple orchards should be done in September. The September and January SOM values correlated with both yield and, more strongly, with yield efficiency. In the combined September and January data, yield and yield efficiency were well correlated.

Table 3: Pearson correlation coefficients (r) between soil alteration index three (AI3), soil organic matter (SOM) content and plant performance parameters and in a ‘Cripps pink’/M7 apple orchard in September 2006 to 20010 and January 2007 to 2011 (n = 20)

Parameters correlated	r	p
September AI3 x January AI3	0.6069	0.0045
September SOM x January SOM	0.9847	<0.0001
September AI3 x September SOM	-0.7761	<0.0001
January AI3 x January SOM	-0.4613	0.0406
September AI3 x stem circumference increase	-0.1760	0.4580
January AI3 x stem circcumference increase	-0.1027	0.6667
September AI3 x percentage increase in stem circumference	-0.0480	0.5733
January AI3 x percentage increase in stem circumference	-0.0515	0.5458
September AI3 x yield	0.5110	0.0213
January AI3 x yield	0.2416	0.3049
September AI3 x yield efficiency	0.6565	0.0017
January AI3 x yield efficiency	0.4160	0.0681
September SOM x yield	-0.5885	0.0063
January SOM x yield	-0.6151	0.0039
September SOM x yield efficiency	-0.8296	<0.0001
January SOM x yield efficiency	-0.8516	<0.0001
Yield x yield efficiency	0.8175	<0.0001

AI3 values determined in drip-line top-soils in September were generally higher, and yields and yield efficiencies were also higher, in maturing ‘Cripps Pink’/ M7 orchards under CON than under ORG management protocols. AI3 correlates with SOM. Both AI3 and SOM can therefore be used as indicators of yield and yield efficiency, although AI3 was more sensitive to intraseasonal changes than SOM. The AI3 value range that corresponds to optimum soil conditions for apple production, as opposed to vegetative growth, must still be determined. Testing of AI3 in commercial apple orchards will commence in September 2014.