Tillage may reduce external hyphal integrity and infectivity of arbuscular mycorrhizal (AM) fungi, and P fertilization inhibits AM development.  The objective of this study was to assess the effects of long-term tillage practices on AM colonization and growth of sunflower (Helianthus anuus L.) grown under two P levels. The field experiment, on silt loam soil, was carried out on a range of tillage practices conducted continuously since 1998: rototill [R], plow and disk [PD], aeration [A], and zero till [Z].  The four tillage treatments were main plots and two levels of P fertilizer (0 and 45 kg P ha^-1) were arranged as sub plots in a split plot design with four replicates.  Sunflower shoots and roots were sampled at the six-leaf stage, ten-leaf stage and before flowering to measure percentage of root colonization and shoot dry weight. Total shoot dry weight and grain yield were also determined after grain maturity.  In general, long-term intensive tillage (R and PD) reduced degree of root colonization, growth of sunflower shoots and grain yield compared to light tillage (A) and no-till (Z).  Fertilization with P negatively influenced AM colonization but increased root and shoot dry weight. This experiment suggests intensive soil disturbance (R and PD) disrupts the external hyphal network resulting in reduction of nutrient uptake by AM fungi and growth sunflower.

 Table 1. Effect of different tillage and P fertilizer rate on sunflower root dry weight and root dry weight (kg ha^-1).

Factor	Level	6-leaf		10-leaf		Pre- flowering
		RDW*	SDW	RDW	SDW	SDW
Tillage	Z	15.6 b**	154.4 a	116 ab	1214 a	5530 b
	A	19.4 a	152.4 a	139 a	1361 a	6531 a
	R	13.5 b	98.2 b	88.3 b	873 b	4979 bc
	PD	11.7 b	96.8 b	87.6 b	932 b	4678 c
P rate (kg/ha)	0	11.7 b	90.2 b	80.9 b	806 b	4641 b
	45	16.5 a	145.1 a	120 a	1245 a	5745 a

^*RDW=root dry weight; SDW=shoot dry weight; Z=Zero till; A=Aeration; R=Rototill; PD=Plow and disk.

^**Means followed by the same letter within the same column and in the same factor are not significantly different (p< 0.05).

 Table 2. Effect of different tillage and P fertilizer rate on percentage of AM colonization in sunflower roots. 

Factor	Level	6-leaf	10-leaf	Pre-flowering
Tillage	Z*	51.0 a**	59.1 a	59.2 a
	A	51.0 a	54.0 a	56.7 ab
	R	36.7 b	43.7 b	44.0 b
	PD	33.0 b	47.1 b	49.1 ab
P rate (kg/ha)	0	45.0 a	54.1 a	54.4 a
	45	39.3 b	47.9 b	49.5 a

^* and ^** represent the same meanings as those in Table 1.