  We provide an analogue of Gundy's decomposition for L1-bounded
non-commutative martingales. An important difference from the classical case is
that for any L1-bounded non-commutative martingale, the decomposition consists
of four martingales. This is strongly related with the row/column nature of
non-commutative Hardy spaces of martingales. As applications, we obtain simpler
proofs of the weak type (1,1) boundedness for non-commutative martingale
transforms and the non-commutative analogue of Burkholder's weak type
inequality for square functions. A sequence (x_n) in a normed space X is called
2-co-lacunary if there exists a bounded linear map from the closed linear span
of (x_n) to l2 taking each x_n to the n-th vector basis of l2. We prove (using
our decomposition) that any relatively weakly compact martingale difference
sequence in L1(M,\tau) whose sequence of norms is bounded away from zero is
2-co-lacunary, generalizing a result of Aldous and Fremlin to non-commutative
L1-spaces.
